The phase relations in the binary system of BN-Mg3N2 were investigated in regions of pressure, P, from 3.0 GPa to 8.0 GPa and temperature, T, up to 1900 K by means of in situ differential thermal analysis (DTA) and X-ray diffraction (XRD) of the quenched products. It was found that the succession in formation of the intermediate compounds Mg3BN3 (HP-phase) and Mg3B2N4 depends on the molar ratios of hexagonal boron nitride (hBN) and Mg3N2 and on the P-T conditions. In the P-T region of cubic boron nitride (cBN) growth, the system has three metastable eutectics such as Mg3N2-hBN, Mg3BN3-hBN and Mg3B2N4-hBN. It was found that eutectic temperatures are pressure dependent. The difference in the lower-temperature limits of cBN growth regions is explained by cBN crystallization from different eutectic melts.