The pressure and temperature regions of cubic BN formation were determined using Fe–Mo–Al and Co–Mo–Al ternary alloys as synthetic solvents of cubic BN. The alloy compositions employed in the present study were (in weight percent) Fe60.14–Mo36.86–Al3 and Co57.6–Mo38.4–Al4. The cubic BN was successfully synthesized at minimum pressure of about 4.4GPa and temperature of about 1250°C. Pressure and temperature of cubic BN synthesis were decreased drastically by small amount of Al addition into Fe–Mo or Co–Mo alloy solvents. The growth of cubic BN was started at the interface between the molten alloy and the source hexagonal BN. In the present study, we proposed that Fe–Mo and Co–Mo work as solvent of B and N atoms and Al acts as a nucleation agent of cubic BN.