PURPOSE: To increase the rate of conversion for synthesizing cubic boron nitride from hexagonal boron nitride.

CONSTITUTION: Hexagonal boron nitride is converted into cubic boron nitride while keeping temp. and pressure each in a region in which cubic boron nitride is stable in the presence of at least one kind of compd. selected among amides and imides of groups Ia and IIa metals optionally in combination with at least one kind of metal selected among groups Ia, IIa, IIIa, VIa, VIIa, VIII, IIb and IIIb metals.