The present invention provides a metal oxide semiconductor (MOS) device, including a substrate, a gate structure on the substrate and a source/drain region disposed in the substrate at one side of the gate structure and in at least a part of an epitaxial structure, wherein the epitaxial structure includes a first buffer layer, which is an un-doped buffer layer, including a bottom portion disposed on a bottom surface of the epitaxial structure and a sidewall portion disposed on a concave sidewall of the epitaxial structure, an epitaxial layer which is encompassed by the first buffer layer, and a semiconductor layer which is disposed between the first buffer layer and the epitaxial layer. The source/drain region is disposed in the epitaxial structure.