A semiconductor device comprises a substrate, a first stress, and a second stress. The substrate has a first-type MOS transistor, an input/output (I/O) second-type MOS transistor, and a core second-type MOS transistor formed thereon. The first-type and the second-type are opposite conductivity types with respect to each other. The first stress layer is only disposed on the first-type MOS transistor, and the second stress layer is different from the first stress, and is only disposed on the core second-type MOS transistor. The I/O second-type MOS transistor is a type of I/O MOS transistor and without not noly the first stress layer but also the second stress layer disposed thereon, the core second-type MOS transistor is a type of core MOS transistor.