A semiconductor device includes: a substrate having a first region and a second region; a first gate structure disposed on the first region, wherein the first gate structure comprises a first high-k dielectric layer, a first work function metal layer, and a first metal layer disposed between the first high-k dielectric layer and the first work function metal layer; and a second gate structure disposed on the second region, wherein the second gate structure comprises a second high-k dielectric layer, a second work function metal layer, and a second metal layer disposed between the second high-k dielectric layer and the second work function metal layer, wherein the thickness of the second metal layer is lower than the thickness of the first metal layer.