diff --git a/demo/mnist/api_train.py b/demo/mnist/api_train.py
index ef8b20a48dc6071cc0e0eff2aa22ca5cb5ba8816..425c5f897a9c254bdae2aa1a6e91a4ce7a69874e 100644
--- a/demo/mnist/api_train.py
+++ b/demo/mnist/api_train.py
@@ -58,11 +58,25 @@ def main():
 
     for _ in xrange(100):
         updater.startPass()
+        outArgs = api.Arguments.createArguments(0)
         train_data_generator = input_order_converter(
             read_from_mnist(train_file))
-        for data_batch in generator_to_batch(train_data_generator, 128):
+        for batch_id, data_batch in enumerate(
+                generator_to_batch(train_data_generator, 256)):
             trainRole = updater.startBatch(len(data_batch))
 
+            def update_callback(param):
+                updater.update(param)
+
+            m.forwardBackward(
+                converter(data_batch), outArgs, trainRole, update_callback)
+
+            cost_vec = outArgs.getSlotValue(0)
+            cost_vec = cost_vec.copyToNumpyMat()
+            cost = cost_vec.sum() / len(data_batch)
+            print 'Batch id', batch_id, 'with cost=', cost
+            updater.finishBatch(cost)
+
         updater.finishPass()
 
     m.finish()
diff --git a/paddle/api/PaddleAPI.h b/paddle/api/PaddleAPI.h
index 165997ba3499f4ee5225aec70ef3dfd0cf1a6d3e..cc49e6a09d5dee41ff47606025fcc492559aa958 100644
--- a/paddle/api/PaddleAPI.h
+++ b/paddle/api/PaddleAPI.h
@@ -799,7 +799,7 @@ public:
 
   void finishPass();
 
-  PassType startBatch(int64_t batchSize);
+  PassType startBatch(size_t batchSize);
 
   void finishBatch(float cost);
 
diff --git a/paddle/api/ParameterUpdater.cpp b/paddle/api/ParameterUpdater.cpp
index e5d07b81782bf5212a043bea8d35c60a8b7ae4fa..fba47620249dbc7543678b3e7e969a21ff32647a 100644
--- a/paddle/api/ParameterUpdater.cpp
+++ b/paddle/api/ParameterUpdater.cpp
@@ -36,8 +36,8 @@ void ParameterUpdater::startPass() { m->updater->startPass(); }
 
 void ParameterUpdater::finishPass() { m->updater->finishPass(); }
 
-PassType ParameterUpdater::startBatch(int64_t batchSize) {
-  return m->updater->startBatch(batchSize);
+PassType ParameterUpdater::startBatch(size_t batchSize) {
+  return m->updater->startBatch((int64_t)batchSize);
 }
 
 void ParameterUpdater::finishBatch(float cost) {