We have investigated the effect of Al on cBN formation in Li3N-hBN system at 5.0 GPa and 1300-1650 degrees C. Regular cBN single crystals of 0.2-0.5 mm in size were obtained. It appears that the presence of Al in hBN powder facilitates the formation of cBN crystals with regular shape, although it does not have any catalytic action for hBN-cBN phase transformation. With increasing Al concentration, the color of cBN changed darker from amber to black and the threshold temperature for cBN formation became higher. X-ray diffraction and Raman spectroscopy indicate that AlN formed by reaction of Li3N and Al and some B liberated in system. Crown Copyright (C) 2010 Published by Elsevier B.V. All rights reserved.