A semiconductor device includes a gate structure on a substrate, an offset spacer adjacent to the gate structure, a main spacer around the offset spacer, a source/drain region adjacent to two sides of the main spacer, a contact etch stop layer (CESL) adjacent to the main spacer, and an interlayer dielectric (ILD) layer around the CESL. Preferably, a dielectric constant of the offset spacer is higher than a dielectric constant of the main spacer.