A novel "in situ phase-selective route" has been developed, and it was used for the synthesis of cubic boron nitride (cBN) under hydrothermal conditions. The results showed that, if H3BO3, NaN3 and N2H4•H2O were mixed at room temperature and ambient pressure, explosion boron nitride (eBN) was the dominant phase in the sample prepared at 300℃ and 10 MPa. On the contrast, if the reactants were separated with each other at room temperature and ambient pressure, and slowly mixed at high temperature and high pressure (namely, 300℃ and 10MPa) conditions, cBN crystals as large as 0.5mm could be synthesized. This method was called "in situ phase-selective route", it should also be an effective method for the synthesis of diamond crystals in solvothermal and hydrothermal solutions.