The present invention provides a method of forming a metal oxide semiconductor (MOS) device comprising a gate structure and an epitaxial structure. The gate structure is disposed on a substrate. The epitaxial structure is disposed in the substrate at two sides of the gate structure and a part thereof serves a source/drain of the MOS, wherein the epitaxial structure comprises: a first buffer layer with a second conductive type, a second buffer layer, and an epitaxial layer with a first conductive type complementary to the second conductive type. The present invention further provides a method of forming the same.