The method of this invention for heat treatment of a Si single crystal grown by the Czochralski method at a speed of pull of not less than 0.8 mm/min., characterized by heat-treating at a temperature in the range of from 1,150.degree. C. to 1,280.degree. C. a wafer cut out of the Si single crystal thereby producing a Si wafer excellent in oxide film dielectric breakdown voltage characteristic due to elimination of crystal defects. Consequently, this invention ensures production of LSI in a high yield.