PURPOSE: To obtain the subject material, capable of readily controlling the grain diameter and having high mechanical strength and thermal conductivity and satisfying fundamental characteristics as a filler material by forming an outer layer of aluminum nitride on the surface of an inner layer composed of alumina.

CONSTITUTION: The objective material is obtained by forming an outer layer of aluminum nitride or sialon on the surface of an inner layer consisting essentially of alumina or alumina silica-based oxide. The maximum length of grains is 1-300μm. The material is prepared by heating, e.g. grains of a substrate composed of 99% alumina for a filler in a mixed Gas of NH3 and LPG at 1450°C and nitriding the surface. Furthermore, the thickness of the outer layer is preferably ≥0.1μm.