A semiconductor device is provided herein, which includes a substrate having a first-type MOS transistor, an input/output (I/O) second-type MOS transistor, and a core second-type MOS transistor formed thereon. The semiconductor device further includes a first stress layer and a second stress layer. The first stress layer is disposed on the first-type MOS transistor, or on the first-type MOS transistor and the I/O second-type MOS transistor. The second stress layer is disposed on the core second-type MOS transistor.