A method for fabricating semiconductor device is disclosed. The method includes the steps of: providing a substrate having a first region, a second region, a third region, and a fourth region; forming a tuning layer on the second region; forming a first work function metal layer on the first region and the tuning layer of the second region; forming a second work function metal layer on the first region, the second region, and the fourth region; and forming a top barrier metal (TBM) layer on the first region, the second region, the third region, and the fourth region.