Provided is a method of fabricating a MOS device including the following steps. A gate structure is formed on a substrate and a first spacer is formed at a sidewall of the gate structure. A first implant process is performed to form source and drain extension regions in the substrate. A spacer material layer is formed on the gate structure, the first spacer and the substrate. A treatment process is performed so that stress from the spacer material layer is applied onto and memorized in a channel between two source and drain extension regions. An anisotropic process is performed to remove a portion of the spacer material so that a second spacer is formed. A second implant process is performed to form source and drain regions in the substrate.