A method for fabricating semiconductor device includes the steps of: forming a fin-shaped structure on a substrate; forming a first gate structure and a second gate structure on the fin-shaped structure and an interlayer dielectric (ILD) layer around the first gate structure and the second gate structure; transforming the first gate structure and the second gate structure into a first metal gate and a second metal gate; forming a hard mask on the first metal gate and the second metal gate; removing part of the hard mask, the second metal gate, and part of the fin-shaped structure to form a trench; and forming a dielectric layer into the trench to form a single diffusion break (SDB) structure.