A method for fabricating semiconductor device includes the steps of: forming a fin-shaped structure on a substrate; forming a shallow trench isolation (STI) around the fin-shaped structure; forming a gate layer on the fin-shaped structure and the STI; removing part of the gate layer, part of the fin-shaped structure, and part of the STI to form a trench; and forming a dielectric layer into the trench to form a single diffusion break (SDB) structure.