This paper deals with the synthesis of ultrafine diamond (UFD) under high pressure and high temperatures generated by negative oxygen balance explosive detonation.The mechanism of synthesis ultrafine diamond by detonation standpoint is presented. The process of UFD establishment is discussed according to the three stages of detonation propagation. The factors that influence the synthetic UFD process are theoretically analysed.A hermetically spheric container, which can bear internal pressure that the detonation of 100 gramme charge, is designed for the experiment. UFD are purified by various means including H2SO4+HNO3 and H2SO4+KMnO4. The results show that H2SO4+HNO3 is more advantageous than other methods. The process of producing UFD and the methods of purification are determined. The charge used in the experiment is a mixture of TNT and RDX; the influences of its density, slenderness ratio on the yield of UFD are experimentally investigated. The experimental results indicate that the yield of UFD increases with increasing the density, slenderness ratio of charge. UFD synthesis is related to the particle size of RDX.The mean particle size of UFD is measured to be 99.6nm. The UFD is characterized by transmisson electron microscopy (TEM) and X-ray diffraction (XRD); the results showed that the UFD commonly exhibit a spherical shape, and it is cubic crystal.