The kinetics of hBN-into-cBN transformation at 6 GPa and 1770, 1880 and 1990 K in the presence of AlN have been studied. The transformation proceeds without liquid phase. It has been established that a limiting stage of the transformation is diffusion of boron and nitrogen atoms in wurtzitic aluminum nitride. Activation energy of the transformation is 170+/-40 kJ/mol. On the basis of our results we conclude that hexagonal BN dissolves in aluminum nitride solid solution and supersaturates it with respect to cBN, and then cubic boron nitride precipitates from the supersaturated solution of BN in AlN. An AlN-BN system phase diagram is proposed.