A method for fabricating a metal oxide semiconductor (MOS) device is described, including following steps. Two recesses are formed in a substrate. A first epitaxy growth process is performed, so as to form a first semiconductor compound layer in each of the recesses. A second epitaxy growth process is performed with an epitaxial temperature lower than 700° C., so as to form a cap layer on each of the first semiconductor compound layers. Each of the cap layers includes a second semiconductor compound layer protruding from a surface of the substrate. The first and the second semiconductor compound layers are composed of a first Group IV element and a second Group IV element, wherein the second Group IV element is a nonsilicon element. The content of the second Group IV element in the second semiconductor compound layers is less than that in the first semiconductor compound layers.