Abstract: A powder of diamond or high-pressure phase boron nitride core particles charged into a coating space as it is dispersed, and a precursor of a coat forming substance allowed to contact and/or impinge against the particles in the powder of core particles so that their surfaces are covered with the coat forming substance, thereby preparing coated diamond or high-pressure phase boron nitride particles which are subsequently sintered. The thusly produced diamond of high-pressure phase boron nitride sinter is composed of coated core particles of high performance that are superhard, uniform, dense and sintered firmly, and which have a controlled microstructure.