Abstract: In order to eliminate unwanted crystal defects generated by an ion implantation, a semiconductor substrate or an epitaxial layer, which is selectively subjected to an impurity ion implantation, is heat-treated in an inert gas atmosphere at 850.degree. to 1050.degree. C. to recrystallize the implanted region. Thereafter, the semiconductor substrate is heat-treated at 900.degree. to 1250.degree. C. in an atmosphere containing oxygen. For eliminating abnormal growth of grain boundaries in a polycrystalline semiconductor layer deposited on an insulating film the semiconductor layer is heat-treated at 900.degree. to 1100.degree. C. in an atmosphere containing oxygen. By applying at least one of these processes to usual fabrication methods, semiconductor devices with high reliabilty such as power MOSFETs will be provided.