diff --git a/paddle/operators/mul_op.cc b/paddle/operators/mul_op.cc
index 5645df6677ee2a5b52ec00f6f215d4af61b9bab8..329ab95327b76ad997ef27257ca6a5f2a12f1379 100644
--- a/paddle/operators/mul_op.cc
+++ b/paddle/operators/mul_op.cc
@@ -66,10 +66,10 @@ class MulOpGrad : public framework::OperatorWithKernel {
     PADDLE_ENFORCE_NOT_NULL(ctx.InputVar("Y"), "Input(Y) should not be null");
     PADDLE_ENFORCE_NOT_NULL(ctx.InputVar(framework::GradVarName("Out")),
                             "Input(Out@GRAD) should not be null");
+    auto x_dims = ctx.Output<Tensor>("X")->dims();
+    auto y_dims = ctx.Output<Tensor>("Y")->dims();
     auto *x_grad = ctx.Output<Tensor>(framework::GradVarName("X"));
     auto *y_grad = ctx.Output<Tensor>(framework::GradVarName("Y"));
-    auto x_dims = ctx.Output<Tensor>(framework::GradVarName("X"))->dims();
-    auto y_dims = ctx.Output<Tensor>(framework::GradVarName("Y"))->dims();
     auto out_dims = ctx.Input<Tensor>(framework::GradVarName("Out"))->dims();
     PADDLE_ENFORCE(x_dims[0] == out_dims[0],
                    "Out@GRAD M X N must equal to X dims 0, M ");
diff --git a/paddle/operators/mul_op.h b/paddle/operators/mul_op.h
index 2afed81842924796b6f54589fb61627917b4f2f4..9bbd0275261e68a86eb033398b50e73143c99e3b 100644
--- a/paddle/operators/mul_op.h
+++ b/paddle/operators/mul_op.h
@@ -53,7 +53,9 @@ class MulGradKernel : public framework::OpKernel {
     auto* dY = ctx.Output<Tensor>(framework::GradVarName("Y"));
     auto* device_context =
         const_cast<platform::DeviceContext*>(ctx.device_context_);
+    // dX = dOut' * Y. dX: M x K, dOut : M x N, Y : K x N
     math::matmul<Place, T>(*dOut, false, *Y, true, 1, dX, 0, device_context);
+    // dY = X' * dOut. dY: K x N, dOut : M x N, X : M x K
     math::matmul<Place, T>(*X, true, *dOut, false, 1, dY, 0, device_context);
   }
 };