With LiH as additive and Mg/Mg3N2 as catalyst hexagonal boron nitride is transformed into the cubic form, pressure and temperature being maintained at 55-64 Kb and 1700-2100℃ respectively. The structure and related properties of crystals thus obtained are identified and tested. Microseopie examination of impurities, defects, growth centers and steps on crystal surfaces indicates that this cubic BN possesses the common features of crystals grown under ordinary conditions. The results of the synthesis which depends closely upon the P-T condition, catalyst composition and catalyst assembly are analysed and discussed. The remarkable effect of the presence of a small amount of LiH in the catalyst on the grain growth is particularly described. Cubic BN crystals larger than 1mm. have been obtained.