A semiconductor device and a manufacturing method thereof are provided. The semiconductor device includes a substrate, a metal-oxide-semiconductor (MOS) transistor, and a dielectric layer. The MOS transistor includes a gate structure formed over the substrate. The dielectric layer is formed aside the gate structure, and the dielectric layer is doped with a strain modulator. An effective lattice constant of the dielectric layer modified by the doping with the strain modulator is different from an effective lattice constant of the dielectric layer prior to the doping.