diff --git a/python/paddle/trainer_config_helpers/layers.py b/python/paddle/trainer_config_helpers/layers.py
index 1564be10ac7fea3ae5fa60f73c2efc4d79975c34..27fba42c70a312845fd4ad867329c77485817f8f 100755
--- a/python/paddle/trainer_config_helpers/layers.py
+++ b/python/paddle/trainer_config_helpers/layers.py
@@ -482,7 +482,7 @@ def table_projection(input, size=0, param_attr=None):
     return proj
 
 
-def identity_projection(input, offset=None):
+def identity_projection(input, offset=None, size=None):
     """
     1. IdentityProjection if offset=None. It performs:
 
@@ -523,8 +523,10 @@ def identity_projection(input, offset=None):
         proj = IdentityProjection(input_layer_name=input.name)
         proj.origin = input
     else:
+        if size is None:
+            size = input.size - offset
         proj = IdentityOffsetProjection(
-            input_layer_name=input.name, offset=offset)
+            input_layer_name=input.name, offset=offset, size=size)
         proj.origin = input
     return proj
 
@@ -2797,7 +2799,7 @@ def concat_layer(input, act=None, name=None, layer_attr=None, bias_attr=None):
     if layer_type == LayerType.CONCAT_LAYER:
         assert not bias_attr
 
-    Layer(
+    layer = Layer(
         name=name,
         type=layer_type,
         inputs=[x.name for x in input] if is_concat_layer else input,
@@ -2805,13 +2807,7 @@ def concat_layer(input, act=None, name=None, layer_attr=None, bias_attr=None):
         bias=ParamAttr.to_bias(bias_attr),
         **ExtraLayerAttribute.to_kwargs(layer_attr))
 
-    sz = 0
-    for each_input in input:
-        if each_input.size is not None:
-            sz += each_input.size
-        else:
-            sz = None
-            break
+    sz = layer.config.size
 
     return LayerOutput(
         name,
@@ -2979,7 +2975,7 @@ def memory(name,
 @layer_support()
 def lstm_step_layer(input,
                     state,
-                    size,
+                    size=None,
                     act=None,
                     name=None,
                     gate_act=None,
@@ -3045,6 +3041,9 @@ def lstm_step_layer(input,
     :return: LayerOutput object.
     :rtype: LayerOutput
     """
+
+    assert size is None or state.size == size
+    size = state.size
     Layer(
         name=name,
         type=LayerType.LSTM_STEP_LAYER,
@@ -3052,7 +3051,7 @@ def lstm_step_layer(input,
         active_gate_type=gate_act.name,
         active_state_type=state_act.name,
         bias=ParamAttr.to_bias(bias_attr),
-        size=size,
+        size=state.size,
         inputs=[input.name, state.name],
         **ExtraLayerAttribute.to_kwargs(layer_attr))