Mechanical properties and cutting performances of high purity polycrystalline cBN (PcBN) compacts synthesized by the direct conversion method from hexagonal BN under high pressure and high temperature have been investigated. The hardness and transverse rupture strength (TRS) of the compacts in the temperature range of 293 to 1473 K depend strongly on the cBN grain size and the amount of residual hBN in the compact. The fine-grained (< 0.5 μm) and high purity (> 99.9%) PcBN compacts synthesized at 7.7 GPa and about 2550 K. has the highest hardness and TRS at any temperature. All high purity PcBN compacts have high thermal conductivity above 300 W/m.K and show excellent thermal crack resistance in high speed wet cutting of gray cast iron and milling of steel, which are difficult for the conventional cutting tool materials. Especially, the fine-grained compact, which has high strength, exhibits excellent cutting performances.