A method of manufacturing a metal oxide semiconductor is provided. The method includes forming an offset spacer and a disposable spacer around the offset spacer. Then, forming a plurality of epitaxial layers outside the disposable spacer and removing the disposable spacer. In addition, the method includes forming a plurality of source/drain extension areas in the substrate outside the offset spacer and the epitaxial layers. Because the source/drain extension areas are formed after the selective epitaxial growth process, the thermal of the selective epitaxial growth process does not damage the source/drain extension areas.