A method for fabricating semiconductor device includes the steps of: providing a substrate having a first region and a second region; forming a first fin-shaped structure on the first region; forming a first gate structure and a second gate structure on the first fin-shaped structure; using a patterned mask to remove the first gate structure and part of the first fin-shaped structure to form a first trench; and forming a first dielectric layer in the first trench to form a first single diffusion break (SDB) structure and around the second gate structure.