A method for fabricating a semiconductor device includes the steps of forming a fin-shaped structure on a substrate, forming a first trench and a second trench in the fin-shaped structure, forming a first dielectric layer in the first trench and the second trench, removing part of the first dielectric layer, forming a second dielectric layer in the first trench and the second trench to form a first single diffusion break (SDB) structure and a second SDB structure, and then forming a gate structure on the fin-shaped structure, the first SDB structure, and the second SDB structure.