A method for forming a metal-oxide-semiconductor (MOS) device includes at least steps of forming a pair of trenches in a substrate at both sides of a gate structure, filling the trenches with a silicon germanium layer by a selective epitaxy growth process, forming a cap layer on the silicon germanium layer by a selective growth process, and forming a pair of source/drain regions by performing an ion implantation process. Hence, the undesirable effects caused by ion implantation can be mitigated.