A method for fabricating semiconductor device includes the steps of: providing a substrate having a first region and a second region; forming a first bottom barrier metal (BBM) layer on the first region and the second region; forming a first work function metal layer on the first BBM layer on the first region and the second region; removing the first work function metal (WFM) layer and part of the first BBM layer on the second region; and forming a diffusion barrier layer on the first WFM layer on the first region and the first BBM layer on the second region.