Abstract: The powder of hexagonal boron nitride is prepared by heat-treating a powdery mixture composed of an oxygen-containing boron compound, e.g. boric acid, and a nitrogen compound, e.g. dicyandiamide and melamine, with admixture of minor amounts of a carbonaceous powder, e.g. carbon black, and/or borax. Further, an alkaline earth metal compound such as calcium carbonate is added either to the powdery mixture of the starting materials before the heat treatment or to the powder after the heat treatment. The h-BN powder has excellent sinterability so that sintered bodies of boron nitride having a high density and high bending strength can readily be prepared by use of a hot press.