diff --git a/paddle/fluid/framework/block_desc.cc b/paddle/fluid/framework/block_desc.cc
index f4bb2f3e2fc2c8cf0376631d1996b395a8bc581a..0b7aaf11746d1931e10ad7e5368d9e053092500e 100644
--- a/paddle/fluid/framework/block_desc.cc
+++ b/paddle/fluid/framework/block_desc.cc
@@ -13,7 +13,11 @@ See the License for the specific language governing permissions and
 limitations under the License. */
 
 #include "paddle/fluid/framework/block_desc.h"
+
 #include <queue>
+#include <unordered_set>
+#include <utility>
+
 #include "paddle/fluid/framework/operator.h"
 #include "paddle/fluid/framework/program_desc.h"
 
@@ -155,6 +159,16 @@ void BlockDesc::RemoveOp(size_t s, size_t e) {
   ops_.erase(ops_.begin() + s, ops_.begin() + e);
 }
 
+void BlockDesc::RemoveOpInternal(const OpDesc *op_desc) {
+  // TODO(minqiyang): make this faster
+  for (auto it = ops_.begin(); it != ops_.end(); ++it) {
+    if (it->get() == op_desc) {
+      ops_.erase(it);
+      break;
+    }
+  }
+}
+
 std::vector<OpDesc *> BlockDesc::AllOps() const {
   std::vector<OpDesc *> res;
   for (const auto &op : ops_) {
@@ -163,20 +177,6 @@ std::vector<OpDesc *> BlockDesc::AllOps() const {
   return res;
 }
 
-void BlockDesc::Clear() {
-  // clear all ops
-  ops_.clear();
-
-  // clear all vars which are not persistable
-  for (auto it = vars_.begin(); it != vars_.end();) {
-    if (it->second->Persistable()) {
-      ++it;
-    } else {
-      vars_.erase(it++);
-    }
-  }
-}
-
 void BlockDesc::Flush() {
   for (auto &op_desc : ops_) {
     op_desc->Flush();
diff --git a/paddle/fluid/framework/block_desc.h b/paddle/fluid/framework/block_desc.h
index e192624a261e1291f1610e8e7e700d99a9d814d2..5c6e421516269a9b9865605400efa772f944a96f 100644
--- a/paddle/fluid/framework/block_desc.h
+++ b/paddle/fluid/framework/block_desc.h
@@ -93,12 +93,12 @@ class BlockDesc {
    */
   void RemoveOp(size_t s, size_t e);
 
+  void RemoveOpInternal(const OpDesc *op_desc);
+
   void RemoveVar(const std::string &name) { vars_.erase(name); }
 
   std::vector<OpDesc *> AllOps() const;
 
-  void Clear();
-
   size_t OpSize() const { return ops_.size(); }
 
   OpDesc *Op(int idx) const { return ops_.at(idx).get(); }
diff --git a/paddle/fluid/framework/python_headers.h b/paddle/fluid/framework/python_headers.h
index 422af19a13683dc9ae6377cac1b1ab2c2ac8f96b..8f9e3fad57f7bb87e78e334e741be23751417a78 100644
--- a/paddle/fluid/framework/python_headers.h
+++ b/paddle/fluid/framework/python_headers.h
@@ -24,3 +24,11 @@ limitations under the License. */
 
 #pragma pop_macro("_XOPEN_SOURCE")
 #pragma pop_macro("_POSIX_C_SOURCE")
+
+#if !defined(PYBIND11_HIDDEN)
+#ifdef _WIN32
+#define PYBIND11_HIDDEN __declspec(dllexport)
+#else
+#define PYBIND11_HIDDEN __attribute__((visibility("hidden")))
+#endif
+#endif
diff --git a/paddle/fluid/imperative/layer.cc b/paddle/fluid/imperative/layer.cc
index aff5cf24be7c41cf58929069768d4fdb34386ae6..012dfc1c7f66027bc5375794e0d70ed78e70e781 100644
--- a/paddle/fluid/imperative/layer.cc
+++ b/paddle/fluid/imperative/layer.cc
@@ -18,6 +18,7 @@
 #include <limits>
 #include <map>
 #include <random>
+#include <unordered_set>
 #include <utility>
 
 #include "paddle/fluid/framework/lod_tensor.h"
@@ -139,6 +140,8 @@ class Autograd {
           }
         }
       }
+
+      ready_op->InvokeBackwardHooks();
     }
   }
 
@@ -156,8 +159,10 @@ class Autograd {
       for (auto it : candidate->pre_ops_) {
         for (OpBase* pre_op : it.second) {
           if (!pre_op) continue;
-          VLOG(5) << "op dep " << candidate->op_desc_->Type() << " <---- "
-                  << it.first << " <---- " << pre_op->op_desc_->Type();
+          VLOG(5) << "op dep " << candidate->op_desc_->Type() << " trace id "
+                  << candidate->trace_id_ << " <---- " << it.first << " <---- "
+                  << pre_op->op_desc_->Type() << " trace id "
+                  << pre_op->trace_id_;
           if (visited.find(pre_op) == visited.end()) {
             visited.insert(pre_op);
             queue.push_back(pre_op);
@@ -211,6 +216,7 @@ std::map<std::string, std::vector<VarBase*>> OpBase::ApplyGrad() {
     return {};
   }
 
+  VLOG(3) << "apply op grad: " << op_desc_->Type();
   std::vector<framework::VariableValueMap> grad_outputs;
   if (backward_id_ > 0) {
     VLOG(3) << "py_layer_grad";
@@ -272,6 +278,22 @@ std::map<std::string, std::vector<VarBase*>> OpBase::ApplyGrad() {
   return input_vars_;
 }
 
+void OpBase::InvokeBackwardHooks() {
+  VLOG(3) << "call backward hooks, hooks num: " << backward_hooks_.size();
+
+  // call backward hooks
+  for (py::object& callable : backward_hooks_) {
+    callable(this);
+  }
+}
+
+void OpBase::RegisterBackwardHooks(const py::object& callable) {
+  VLOG(3) << "Register backward hooks " << trace_id_;
+
+  // TODO(minqiyang): check the callable format
+  backward_hooks_.push_back(callable);
+}
+
 void VarBase::RunBackward() {
   if (!pre_op_) return;
 
diff --git a/paddle/fluid/imperative/layer.h b/paddle/fluid/imperative/layer.h
index bbf614831ca817031b209fffec043495ea24d10f..7a9f33dc1e6cbc0c3ec1e649906fb0a8de047189 100644
--- a/paddle/fluid/imperative/layer.h
+++ b/paddle/fluid/imperative/layer.h
@@ -123,22 +123,32 @@ class VarBase {
 
  private:
   VarBase(framework::Variable* var, VarBase* grad, bool stop_gradient)
-      : var_desc_(nullptr),
+      : name_(),
+        var_desc_(nullptr),
         var_(var),
         grads_(grad),
+        block_(nullptr),
+        persistable_(false),
         stop_gradient_(stop_gradient),
         pre_op_(nullptr),
+        pre_op_out_name_(),
         pre_op_out_idx_(-1) {}
 
  public:
   virtual ~VarBase() {
+    // TODO(minqiyang): remove var desc from block desc
     if (var_) {
       delete var_;
+      var_ = nullptr;
     }
 
     if (grads_) {
       delete grads_;
+      grads_ = nullptr;
     }
+
+    pre_op_ = nullptr;
+    pre_op_out_idx_ = -1;
   }
 
   inline OpBase* PreOp() const { return pre_op_; }
@@ -151,6 +161,14 @@ class VarBase {
 
   void RunBackward();
 
+  inline void ResetPreOp(OpBase* op) {
+    if (op == pre_op_) {
+      // clear pre_op info when op equals to var's pre_op
+      pre_op_ = nullptr;
+      pre_op_out_idx_ = -1;
+    }
+  }
+
   void TrackPreOp(OpBase* pre_op, const std::string& pre_op_out_name,
                   int pre_op_out_idx, bool pre_op_stop_gradient) {
     pre_op_ = pre_op;
@@ -184,11 +202,15 @@ class VarBase {
     return string::Sprintf("%s@IGrad", var_desc_->Name());
   }
 
+  std::string name_;
   framework::VarDesc* var_desc_;
 
   framework::Variable* var_;
   VarBase* grads_;
 
+  framework::BlockDesc* block_;
+  bool persistable_;
+
  private:
   bool stop_gradient_;
   OpBase* pre_op_;
@@ -199,15 +221,27 @@ class VarBase {
 /* The wrapper for OpDesc which holds a OpDesc and a OpDesc of its
  * gradient. This object should be managed totally by Python intepreter.
  */
-class OpBase {
+class PYBIND11_HIDDEN OpBase {
  public:
   OpBase()
       : op_desc_(nullptr),
         forward_id_(-1),
         backward_id_(-1),
-        place_(platform::CPUPlace()) {}
+        trace_id_(-1),
+        place_(platform::CPUPlace()),
+        backward_hooks_() {}
 
   virtual ~OpBase() {
+    // TODO(minqiyang): remove op_desc from block_desc in tracer
+    //
+    // reset all output vars' pre op
+    for (auto iter : output_vars_) {
+      for (VarBase* var : iter.second) {
+        var->ResetPreOp(this);
+      }
+    }
+
+    // release resource
     for (framework::OpDesc* desc : grad_op_descs_) {
       delete desc;
     }
@@ -215,6 +249,10 @@ class OpBase {
 
   std::map<std::string, std::vector<VarBase*>> ApplyGrad();
 
+  void RegisterBackwardHooks(const py::object& callable);
+
+  void InvokeBackwardHooks();
+
   // One of `op_desc_` or `forward_id_` is set, not both.
   // For pure python PyLayer, use `forward_id_`, otherwise, use op_desc_.
   framework::OpDesc* op_desc_;
@@ -225,6 +263,7 @@ class OpBase {
   // Note: each fwd op corresponds to a vector of bwd ops.
   std::vector<framework::OpDesc*> grad_op_descs_;
   int backward_id_;
+  int trace_id_;
 
   platform::Place place_;
 
@@ -239,6 +278,8 @@ class OpBase {
   std::vector<framework::VariableValueMap> grad_output_vars_;
 
   framework::BlockDesc* block_;
+
+  std::vector<py::object> backward_hooks_;
 };
 
 class Layer {
diff --git a/paddle/fluid/imperative/tracer.cc b/paddle/fluid/imperative/tracer.cc
index 67fe6da66d3a74e0fd1d463ec534dae38bcbd988..0cb1676372fdd35a762e897d269550f2d1e1ac36 100644
--- a/paddle/fluid/imperative/tracer.cc
+++ b/paddle/fluid/imperative/tracer.cc
@@ -14,7 +14,10 @@
 
 #include "paddle/fluid/imperative/tracer.h"
 
+#include <memory>
 #include <set>
+#include <unordered_map>
+#include <unordered_set>
 
 #include "paddle/fluid/operators/math/math_function.h"
 #include "paddle/fluid/platform/device_context.h"
@@ -110,7 +113,8 @@ std::set<std::string> Tracer::Trace(OpBase* op, const VarBasePtrMap& inputs,
   std::map<std::string, VarBase*> vars;
 
   framework::OpDesc* op_desc = op->op_desc_;
-  VLOG(3) << "tracer tracing " << op_desc->Type();
+  VLOG(3) << "tracer tracing " << op_desc->Type() << " trace id "
+          << op->trace_id_;
   op_desc->InferShape(*block);
   op_desc->InferVarType(block);
 
@@ -133,11 +137,13 @@ std::set<std::string> Tracer::Trace(OpBase* op, const VarBasePtrMap& inputs,
       if (inp->PreOp() && !inp->IsStopGradient()) {
         op->pre_ops_[it.first].push_back(inp->PreOp());
         op->pre_ops_out_idx_[it.first].push_back(inp->PreOpOutIdx());
+        VLOG(3) << "add pre op " << inp->PreOp()->op_desc_->Type();
       } else {
         op->pre_ops_[it.first].push_back(nullptr);
       }
       VLOG(3) << "input vname " << inp->var_desc_->Name() << " "
-              << inp->var_->IsInitialized();
+              << inp->var_->IsInitialized() << " stop_gradient "
+              << inp->IsStopGradient();
     }
   }
 
@@ -189,6 +195,7 @@ std::set<std::string> Tracer::Trace(OpBase* op, const VarBasePtrMap& inputs,
 
     op->grad_input_vars_.resize(op->grad_op_descs_.size());
     op->grad_output_vars_.resize(op->grad_op_descs_.size());
+
     for (size_t i = 0; i < op->grad_op_descs_.size(); ++i) {
       framework::OpDesc* grad_op_desc = op->grad_op_descs_[i];
       for (auto it : grad_op_desc->Inputs()) {
@@ -201,7 +208,6 @@ std::set<std::string> Tracer::Trace(OpBase* op, const VarBasePtrMap& inputs,
             PADDLE_ENFORCE(fwd_var_it != vars.end());
             // Forward inputs or outputs.
             grad_in_vars.push_back(fwd_var_it->second->var_);
-            vars_saved_for_backward.insert(it.first);
           } else {
             VarBase* var = vars[var_it->second];
             if (!var->grads_->var_->IsInitialized()) {
@@ -211,6 +217,8 @@ std::set<std::string> Tracer::Trace(OpBase* op, const VarBasePtrMap& inputs,
             // Douts.
             grad_in_vars.push_back(var->grads_->var_);
           }
+
+          vars_saved_for_backward.insert(it.first);
         }
       }
 
diff --git a/paddle/fluid/pybind/imperative.h b/paddle/fluid/pybind/imperative.h
index f947b743f99d5d4994b1a87f89fd6815357d8125..8c48b2a7153c566930a074bd0bab1f054c13c2d5 100644
--- a/paddle/fluid/pybind/imperative.h
+++ b/paddle/fluid/pybind/imperative.h
@@ -33,7 +33,7 @@ class Layer : public imperative::Layer {
   }
 };
 
-class PyOpBase : public imperative::OpBase {
+class PYBIND11_HIDDEN PyOpBase : public imperative::OpBase {
  public:
   using imperative::OpBase::OpBase;  // Inherit constructors
 };
diff --git a/paddle/fluid/pybind/protobuf.cc b/paddle/fluid/pybind/protobuf.cc
index 48fe445b7d01287c37bcf7d4811f687785ca78d5..e729be4a95a58510f1e0162af4216feaa400d971 100644
--- a/paddle/fluid/pybind/protobuf.cc
+++ b/paddle/fluid/pybind/protobuf.cc
@@ -189,8 +189,6 @@ void BindBlockDesc(pybind11::module *m) {
              return self.HasVar(name);
            },
            pybind11::return_value_policy::reference)
-      .def("_clear_block", [](pd::BlockDesc &self) { return self.Clear(); },
-           pybind11::return_value_policy::reference)
       .def("_rename_var",
            [](pd::BlockDesc &self, const pybind11::bytes &byte_name,
               const pybind11::bytes &byte_name_new) {
diff --git a/paddle/fluid/pybind/pybind.cc b/paddle/fluid/pybind/pybind.cc
index 6244e1f9ef992a61b525ee5db31191994dbe9012..cf59ff6d3b97a4be5d87f1185acc6173b5d501b2 100644
--- a/paddle/fluid/pybind/pybind.cc
+++ b/paddle/fluid/pybind/pybind.cc
@@ -177,6 +177,23 @@ PYBIND11_MODULE(core, m) {
            py::return_value_policy::take_ownership)
       .def("value", [](const imperative::VarBase &self) { return self.var_; },
            py::return_value_policy::reference)
+      .def_property("name",
+                    [](const imperative::VarBase &self) { return self.name_; },
+                    [](imperative::VarBase &self, const std::string &name) {
+                      self.name_ = name;
+                    })
+      .def_property("block",
+                    [](const imperative::VarBase &self) { return self.block_; },
+                    [](imperative::VarBase &self, framework::BlockDesc *block) {
+                      self.block_ = block;
+                    },
+                    py::return_value_policy::reference)
+      .def_property(
+          "persistable",
+          [](const imperative::VarBase &self) { return self.persistable_; },
+          [](imperative::VarBase &self, const bool persistable) {
+            self.persistable_ = persistable;
+          })
       .def_property(
           "desc",
           [](const imperative::VarBase &self) { return self.var_desc_; },
@@ -193,6 +210,10 @@ PYBIND11_MODULE(core, m) {
 
   py::class_<imperative::OpBase, PyOpBase>(m, "OpBase", R"DOC()DOC")
       .def(py::init<>())
+      .def("register_backward_hooks",
+           [](imperative::OpBase &self, const py::object &callable) {
+             self.RegisterBackwardHooks(callable);
+           })
       .def_property(
           "desc", [](const imperative::OpBase &self) { return self.op_desc_; },
           [](imperative::OpBase &self, framework::OpDesc *op_desc) {
@@ -201,6 +222,16 @@ PYBIND11_MODULE(core, m) {
             }
           },
           py::return_value_policy::reference)
+      .def_property("_trace_id",
+                    [](const imperative::OpBase &self) {
+                      pybind11::gil_scoped_release release;
+                      return self.trace_id_;
+                    },
+                    [](imperative::OpBase &self, int trace_id) {
+                      pybind11::gil_scoped_release release;
+                      self.trace_id_ = trace_id;
+                    },
+                    py::return_value_policy::reference)
       .def_property(
           "forward_id",
           [](const imperative::OpBase &self) { return self.forward_id_; },
@@ -413,11 +444,11 @@ PYBIND11_MODULE(core, m) {
            Set LoD of the LoDTensor according to recursive sequence length.
 
            For example, if recursive_sequence_lengths=[[2, 3]], meaning that
-           there are two sequences with length 2 and 3 respectively, the 
-           corresponding lod would be [[0, 2, 2+3]], i.e, [[0, 2, 5]].  
+           there are two sequences with length 2 and 3 respectively, the
+           corresponding lod would be [[0, 2, 2+3]], i.e, [[0, 2, 5]].
 
            Args:
-                recursive_sequence_lengths (List[List[int]]): sequence lengths. 
+                recursive_sequence_lengths (List[List[int]]): sequence lengths.
            )DOC")
       .def("lod",
            [](LoDTensor &self) -> std::vector<std::vector<size_t>> {
@@ -448,7 +479,7 @@ PYBIND11_MODULE(core, m) {
            Return the sequence length of the LoDTensor corresponding to LoD.
 
            Returns:
-               out (List[List[int]): the sequence lengths. 
+               out (List[List[int]): the sequence lengths.
            )DOC")
       .def("has_valid_recursive_sequence_lengths",
            [](LoDTensor &self) -> bool {
@@ -599,29 +630,29 @@ All parameter, weight, gradient are variables in Paddle.
            },
            py::arg("name"),
            R"DOC(
-           Find or create variable named :code:`name` in the current scope. 
+           Find or create variable named :code:`name` in the current scope.
 
-           If the variable named :code:`name` does not exist in the 
+           If the variable named :code:`name` does not exist in the
            current scope, the variable would be created. Otherwise,
-           return the existing variable. 
+           return the existing variable.
 
            Args:
-               name (str): the variable name.  
-          
+               name (str): the variable name.
+
            Returns:
-               out (core.Variable): the found or created variable. 
+               out (core.Variable): the found or created variable.
            )DOC",
            py::return_value_policy::reference)
       .def("find_var", &Scope::FindVar, py::arg("name"),
            R"DOC(
-           Find variable named :code:`name` in the current scope or 
+           Find variable named :code:`name` in the current scope or
            its parent scope. Return None if not found.
-        
+
            Args:
                name (str): the variable name.
-            
+
            Returns:
-               out (core.Variable|None): the found variable or None.   
+               out (core.Variable|None): the found variable or None.
            )DOC",
            py::return_value_policy::reference)
       .def("new_scope", [](Scope &self) -> Scope * { return &self.NewScope(); },
@@ -645,7 +676,7 @@ All parameter, weight, gradient are variables in Paddle.
         },
         R"DOC(
         Create a new scope.
-        
+
         Returns:
             out (core._Scope): the created scope.
         )DOC",
diff --git a/python/paddle/fluid/framework.py b/python/paddle/fluid/framework.py
index 73a94821f221fb914adf7d4215b30142c1172ee7..54f4bc5371e7798b37a4fb03ea473f96b40f81d0 100644
--- a/python/paddle/fluid/framework.py
+++ b/python/paddle/fluid/framework.py
@@ -393,6 +393,9 @@ class Variable(object):
             if not self._ivar:
                 self._ivar = core.VarBase(stop_gradient)
             self._ivar.desc = self.desc
+            self._ivar.block = block.desc
+            self._ivar.name = name
+            self._ivar.persistable = persistable
             if persistable:
                 self.block.vars[name] = self
         else:
@@ -721,7 +724,9 @@ class Operator(object):
                 out_arg_names = []
                 for arg in out_args:
                     out_arg_names.append(cpt.to_text(arg.name))
-                    arg.op = self
+                    # TODO(minqiyang): could we remove variable's op in static mode?
+                    if not _in_imperative_mode():
+                        arg.op = self
                 self.desc.set_output(out_proto.name, out_arg_names)
 
         if op_attrs is not None:
@@ -1200,15 +1205,6 @@ class Block(object):
         else:
             raise ValueError("Var {0} is not found recursively".format(name))
 
-    def _clear_block(self):
-        # TODO(minqiyang): move this to backward_hooks
-        self.desc._clear_block()
-
-        for name in self.vars.keys():
-            assert self.vars[name].persistable
-
-        del self.ops[:]
-
     def all_parameters(self):
         return list(self.iter_parameters())
 
@@ -1345,26 +1341,13 @@ class Block(object):
             #
             # TODO(minqiyang): add op stop_gradient support in static mode too.
             # currently, we only support stop_gradient in imperative mode.
-            self._trace_op(op, kwargs.get("stop_gradient", False))
-        self.ops.append(op)
+            _imperative_tracer().trace_op(op,
+                                          kwargs.get("stop_gradient", False))
+        else:
+            self.ops.append(op)
 
         return op
 
-    def _trace_op(self, op, stop_gradient=False):
-        backward_refs = _imperative_tracer().trace(
-            op.iop, op.inputs, op.outputs, self.desc,
-            _imperative_current_expected_place_, stop_gradient)
-
-        # TODO(minqiyang): support backward_hooks to eager remove backward_refs
-        op.backward_refs = defaultdict(list)
-        for k, v in six.iteritems(op.inputs):
-            if k in backward_refs:
-                op.backward_refs[k] = op.inputs[k]
-
-        for k, v in six.iteritems(op.outputs):
-            if k in backward_refs:
-                op.backward_refs[k] = op.outputs[k]
-
     def _insert_op(self, index, *args, **kwargs):
         """
         Insert a Operator according to the giving arguments.
@@ -1417,9 +1400,11 @@ class Block(object):
             inputs=kwargs.get("inputs", None),
             outputs=kwargs.get("outputs", None),
             attrs=kwargs.get("attrs", None))
-        self.ops.insert(0, op)
         if _in_imperative_mode():
-            self._trace_op(op, kwargs.get("stop_gradient", False))
+            _imperative_tracer().trace_op(op,
+                                          kwargs.get("stop_gradient", False))
+        else:
+            self.ops.insert(0, op)
         return op
 
     def _sync_with_cpp(self):
diff --git a/python/paddle/fluid/imperative/__init__.py b/python/paddle/fluid/imperative/__init__.py
index 54dc794ea6392fac6f266477fe045b37001a8666..034a11e0a6049c17800c8fd5aab5bc2291320169 100644
--- a/python/paddle/fluid/imperative/__init__.py
+++ b/python/paddle/fluid/imperative/__init__.py
@@ -23,7 +23,11 @@ from .layers import *
 from . import nn
 from .nn import *
 
+from . import tracer
+from .tracer import *
+
 __all__ = []
 __all__ += layers.__all__
 __all__ += base.__all__
 __all__ += nn.__all__
+__all__ += tracer.__all__
diff --git a/python/paddle/fluid/imperative/base.py b/python/paddle/fluid/imperative/base.py
index d4525233cc681720404770ef1d0c5d3006607a2e..174f138bfa2d3cfaa433c3235c2b0f9a5650e756 100644
--- a/python/paddle/fluid/imperative/base.py
+++ b/python/paddle/fluid/imperative/base.py
@@ -16,6 +16,7 @@ import numpy as np
 
 from paddle.fluid import core
 from paddle.fluid import framework
+from .tracer import Tracer
 
 __all__ = ['enabled', 'guard', 'to_variable']
 
@@ -28,7 +29,7 @@ def enabled():
 def guard(place=None):
     train = framework.Program()
     startup = framework.Program()
-    tracer = core.Tracer(train.current_block().desc)
+    tracer = Tracer(train.current_block().desc)
 
     if place is None:
         if core.is_compiled_with_cuda():
diff --git a/python/paddle/fluid/imperative/tracer.py b/python/paddle/fluid/imperative/tracer.py
new file mode 100644
index 0000000000000000000000000000000000000000..1064ad63e7103acde9bb8106b7791441ce68849b
--- /dev/null
+++ b/python/paddle/fluid/imperative/tracer.py
@@ -0,0 +1,67 @@
+# Copyright (c) 2018 PaddlePaddle Authors. All Rights Reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+from __future__ import print_function
+
+import six
+
+from collections import defaultdict
+from paddle.fluid import core
+from paddle.fluid import framework
+
+__all__ = ['Tracer']
+
+
+def release_op(op):
+    del framework._imperative_tracer()._ops[op._trace_id]
+
+
+class Tracer(core.Tracer):
+    """
+    Python wrapper of imperative tracer
+    """
+
+    def __init__(self, block):
+        super(Tracer, self).__init__(block)
+
+        self._ops = defaultdict()
+        self._trace_id = 0
+
+    def trace_op(self, op, stop_gradient=False):
+        # record op's trace id
+        op.iop._trace_id = self._trace_id
+
+        # trace op and save it
+        backward_refs = self.trace(op.iop, op.inputs, op.outputs, op.block.desc,
+                                   framework._current_expected_place(),
+                                   stop_gradient)
+
+        if not stop_gradient:
+            self._trace_id += 1
+            self._ops[op.iop._trace_id] = op
+
+            # register backward hooks and variables if needed
+            if len(backward_refs) > 0:
+                op.iop.register_backward_hooks(release_op)
+
+                # TODO(minqiyang): remove all inputs and outputs after seperate
+                # var and grad
+                op.backward_refs = defaultdict(list)
+                for k, v in six.iteritems(op.inputs):
+                    if k in backward_refs:
+                        op.backward_refs[k] = op.inputs[k]
+
+                for k, v in six.iteritems(op.outputs):
+                    if k in backward_refs:
+                        op.backward_refs[k] = op.outputs[k]
diff --git a/python/paddle/fluid/initializer.py b/python/paddle/fluid/initializer.py
index e8341be28683a25971a53a37c70533a16add1593..190e7b5608a0cdf156b449e919e108a0917a0980 100644
--- a/python/paddle/fluid/initializer.py
+++ b/python/paddle/fluid/initializer.py
@@ -19,6 +19,7 @@ import numpy as np
 from .wrapped_decorator import signature_safe_contextmanager
 from .core import VarDesc
 from . import unique_name
+from .imperative import base as imperative_base
 
 __all__ = [
     'Constant', 'Uniform', 'Normal', 'TruncatedNormal', 'Xavier', 'Bilinear',
@@ -165,7 +166,8 @@ class ConstantInitializer(Initializer):
                 'force_cpu': self._force_cpu or force_init_on_cpu()
             },
             stop_gradient=True)
-        var.op = op
+        if not imperative_base.enabled():
+            var.op = op
         return op
 
 
@@ -244,7 +246,8 @@ class UniformInitializer(Initializer):
                 attrs={"in_dtype": out_var.dtype,
                        "out_dtype": var.dtype})
 
-        var.op = op
+        if not imperative_base.enabled():
+            var.op = op
         return op
 
 
@@ -322,7 +325,8 @@ class NormalInitializer(Initializer):
                 outputs={"Out": var},
                 attrs={"in_dtype": out_var.dtype,
                        "out_dtype": var.dtype})
-        var.op = op
+        if not imperative_base.enabled():
+            var.op = op
         return op
 
 
@@ -400,7 +404,8 @@ class TruncatedNormalInitializer(Initializer):
                 outputs={"Out": var},
                 attrs={"in_dtype": out_var.dtype,
                        "out_dtype": var.dtype})
-        var.op = op
+        if not imperative_base.enabled():
+            var.op = op
         return op
 
 
@@ -505,7 +510,8 @@ class XavierInitializer(Initializer):
                     "seed": self._seed
                 },
                 stop_gradient=True)
-        var.op = op
+        if not imperative_base.enabled():
+            var.op = op
         return op
 
 
@@ -605,7 +611,8 @@ class MSRAInitializer(Initializer):
                     "seed": self._seed
                 },
                 stop_gradient=True)
-        var.op = op
+        if not imperative_base.enabled():
+            var.op = op
         return op
 
 
@@ -703,7 +710,8 @@ class BilinearInitializer(Initializer):
                 'shape': list(shape),
                 value_name: values
             })
-        var.op = op
+        if not imperative_base.enabled():
+            var.op = op
         return op
 
 
@@ -761,7 +769,8 @@ class NumpyArrayInitializer(Initializer):
                 value_name: values
             },
             stop_gradient=True)
-        var.op = op
+        if not imperative_base.enabled():
+            var.op = op
         return op
 
 
diff --git a/python/paddle/fluid/tests/unittests/test_imperative.py b/python/paddle/fluid/tests/unittests/test_imperative_basic.py
similarity index 100%
rename from python/paddle/fluid/tests/unittests/test_imperative.py
rename to python/paddle/fluid/tests/unittests/test_imperative_basic.py
diff --git a/python/paddle/fluid/tests/unittests/test_imperative_optimizer.py b/python/paddle/fluid/tests/unittests/test_imperative_optimizer.py
index 0d0a3bbe0bd47fe0e01761f8b42c92b884a5680a..7afbf61472a3d09ba5e34731d3a3ebbb8076e310 100644
--- a/python/paddle/fluid/tests/unittests/test_imperative_optimizer.py
+++ b/python/paddle/fluid/tests/unittests/test_imperative_optimizer.py
@@ -12,6 +12,8 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+from __future__ import print_function
+
 import contextlib
 import unittest
 import numpy as np
@@ -142,8 +144,6 @@ class TestImperativeMnist(unittest.TestCase):
                     sgd.minimize(avg_loss)
                     mnist.clear_gradients()
 
-                    fluid.default_main_program().global_block()._clear_block()
-
                     dy_param_value = {}
                     for param in mnist.parameters():
                         dy_param_value[param.name] = param._numpy()
diff --git a/python/paddle/fluid/tests/unittests/test_imperative_ptb_rnn.py b/python/paddle/fluid/tests/unittests/test_imperative_ptb_rnn.py
index c8e42d5ede57896b0d5c09a2334709ced2d16a3f..878c27d9344111d18e1ff27a1d4f41f8ae0df4b0 100644
--- a/python/paddle/fluid/tests/unittests/test_imperative_ptb_rnn.py
+++ b/python/paddle/fluid/tests/unittests/test_imperative_ptb_rnn.py
@@ -243,7 +243,9 @@ class TestImperativePtbRnn(unittest.TestCase):
             dy_loss = None
             last_hidden = None
             last_cell = None
-            for i in range(2):
+            batch_num = 50
+
+            for i in range(batch_num):
                 x_data = np.arange(12).reshape(4, 3).astype('int64')
                 y_data = np.arange(1, 13).reshape(4, 3).astype('int64')
                 x_data = x_data.reshape((-1, num_steps, 1))
@@ -302,7 +304,7 @@ class TestImperativePtbRnn(unittest.TestCase):
             static_loss_value = None
             static_last_cell_value = None
             static_last_hidden_value = None
-            for i in range(2):
+            for i in range(batch_num):
                 x_data = np.arange(12).reshape(4, 3).astype('int64')
                 y_data = np.arange(1, 13).reshape(4, 3).astype('int64')
                 x_data = x_data.reshape((-1, num_steps, 1))
diff --git a/python/paddle/fluid/tests/unittests/test_imperative_resnet.py b/python/paddle/fluid/tests/unittests/test_imperative_resnet.py
index 4892495e1108e6d2a7e96cab88dc7668e360d79f..94ac3933151ac612ea9d308f0e28c73f0c067abf 100644
--- a/python/paddle/fluid/tests/unittests/test_imperative_resnet.py
+++ b/python/paddle/fluid/tests/unittests/test_imperative_resnet.py
@@ -231,7 +231,7 @@ class TestImperativeResnet(unittest.TestCase):
         seed = 90
 
         batch_size = train_parameters["batch_size"]
-        batch_num = 2
+        batch_num = 20
         with fluid.imperative.guard():
             fluid.default_startup_program().random_seed = seed
             fluid.default_main_program().random_seed = seed
@@ -286,8 +286,6 @@ class TestImperativeResnet(unittest.TestCase):
                 optimizer.minimize(avg_loss)
                 resnet.clear_gradients()
 
-                fluid.default_main_program().global_block()._clear_block()
-
                 dy_param_value = {}
                 for param in resnet.parameters():
                     dy_param_value[param.name] = param._numpy()