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; removing part of the first fin-shaped structure to form a first trench; forming a dielectric layer in the first trench, wherein the dielectric layer comprises silicon oxycarbonitride (SiOCN); and planarizing the dielectric layer to form a first single diffusion break (SDB) structure.