diff --git a/paddle/fluid/operators/math/concat.cc b/paddle/fluid/operators/math/concat.cc
index 14964fc62af6dd947e49a2054511780a1bb20cf2..55c8a472aca7fe700ef6a3f96bed1496d7b12b80 100644
--- a/paddle/fluid/operators/math/concat.cc
+++ b/paddle/fluid/operators/math/concat.cc
@@ -93,10 +93,10 @@ class ConcatGradFunctor<platform::CPUDeviceContext, T> {
     auto cpu_place = boost::get<platform::CPUPlace>(context.GetPlace());
 
     // computation
-    for (size_t k = 0; k < input_rows; ++k) {
+    for (int k = 0; k < input_rows; ++k) {
       const T* src_ptr = input.data<T>() + k * input_cols;
       int col_idx = 0;
-      for (int j = 0; j < num; ++j) {
+      for (size_t j = 0; j < num; ++j) {
         int col_len = output_cols[j];
         auto* out_tensor = outputs->at(j);
         if (out_tensor != nullptr) {