An epitaxial wafer includes a base material made of sapphire-SiC single crystal or the like, a III nitride underfilm including at least Al epitaxially grown on the base material and a GaN film, preferably having a thickness of 50 .ANG. or more, formed on the underfilm. In fabricating III nitride films on the epitaxial wafer, the oxidized surface layer of the GaN film is removed through an etching process, and subsequently, another III nitride film is formed thereon.