Abstract: Polycrystalline cubic boron nitride was synthesized by direct phase transition at 9~15 GPa and 1 500~2 100 ℃ using hexagonal boron nitride as initial material. The microstructure and mechanical properties of polycrystalline cubic boron nitride bulk materials were characterized by X-ray diffractometer, scanning electron microscope and Vickers hardness tester. The results show that the pure phase polycrystalline cubic boron nitride can be prepared under suitable temperature and pressure with the minimum grain size about 70 nm and the maximum more than 10 μm. At the same temperature, the grain size of polycrystalline cubic boron nitride bulk decreases with the increase of synthesis pressure, and the hardness increases with the increase of synthesis pressure with the highest hardness 64.45 GPa.