A semiconductor device having metal gate includes a first metal gate structure and a second metal gate structure disposed in a first device region and in a second device region on a substrate respectively. The first metal gate structure includes a gate insulating layer, a first bottom barrier layer, a top barrier layer, and a metal layer disposed on the substrate in order, wherein the top barrier layer is directly in contact with the first bottom barrier layer. The second metal gate structure includes the gate insulating layer, a second bottom barrier layer, the top barrier layer, and the metal layer on the substrate in order, wherein the top barrier layer is directly in contact with the second bottom barrier layer. The first bottom barrier layer and the second bottom barrier layer have different impurity compositions.