Abstract: Under the high temperature and high pressure, the hexagonal graphite crystals can change into the tetragonal diamond crystals. Using the T621A graphite as carbon resources, which contains less ashes and moderate vesicles distributing equally, the alloy of Ni 70 per cents, Mn 25 per cents and Co 5 per cents, as catalyst,which contains a few impurities, and the excellent pyrophyllite as pressure conductive medium,under the conditions of high temperature, moderate pressure and long time, the synthetic diamond crystals are of high strength, coarse grain and good quality.