Provided is a method of fabricating a MOS device including the following steps. At least one gate structure is formed on a substrate, wherein the gate structure includes a gate conductive layer and a hard mask layer disposed on the gate conductive layer. A first implant process is performed to form source and drain extension regions in the substrate, wherein the gate conductive layer is covered by the hard mask layer. A process is of removing the hard mask layer is performed to expose the surface of the gate conductive layer. A second implant process is performed to form pocket doped regions in the substrate, wherein the gate conductive layer is not covered by the hard mask layer.