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.