Single phase sintered compact of cBN (polycrystalline cBN) may be prepared under high pressure and temperature (>5.2GPa, >1350°C) from hBN compact carrying a small amount of Mg₃BN₃ as a catalyst. This work investigated the influence of the amount of Mg₃BN₃ added to the hBN compact on the microstructure and the thermal conductivity of the prepared polycrystalline cBN. It was established that by controlling the amount of Mg₃BN₃ (-1.0mol%) it was possible to prepare polycrystalline cBN which had a high thermal conductivity, approximately 6W/cm°C at room temperature. In addition, it was found that a small amount of water diffused into the hBN compact containing Mg₃BN₃ would suppress abnormal growth of cBN particles. Thus, by adding 0.2wt% of water to the hBN compact containing 1.0mol% of Mg₃BN₃, a homogeneous cBN sintered compact having regular particles (5-8μm) and with high thermal conductivity (-7W/cm°C) could be consistently prepared.