Corundum coated with a silicone compound containing a silicone resin, a resin composition containing the corundum, and an electronic device using an electronic part or semiconductor element, which transmits heat through the resin composition, by which a good affinity between a resin and corundum is obtained and a kneaded product assured of a low viscosity, a uniform dispersibility and when a curable resin is used as the matrix resin, a high curing rate, is yielded so that the handling and moldability of the resin composition can be enhanced.