A method for fabricating semiconductor device includes the steps of: forming a first fin-shaped structure on a substrate; forming a first single diffusion break (SDB) structure in the first fin-shaped structure; forming a first gate structure on the first SDB structure and a second gate structure on the first fin-shaped structure; forming an interlayer dielectric (ILD) layer around the first gate structure and the second gate structure; forming a patterned mask on the first gate structure; and performing a replacement metal gate (RMG) process to transform the second gate structure into a metal gate.