Abstract: Under high temperature (1 000～1 600 K) and high pressure (5.2 GPa) with Ni70Mn25Co5 (100#) being the catalyst, the growing behavior of nanometric diamond particles (2～10 nm) synthesized by the detonation of the explosives has been investigated. The effects of growing conditions on the microstructure and property of sintered nanometric diamond were studied. The samples compressed with nanometric diamond powder and Ni70Mn25Co5 (100#) alloy powder and the disks of Ni70Mn25Co5 (100#) alloy were put in the synthesis reaction cell on alternate. The results indicate that the nanometric diamond powder did not grow, but convert to graphite under high temperature and high pressure. Under the same high pressure and holding time, the degree of graphitization of nanometric diamond powders begin to decrease with the temperature decreasing, and nanometric diamond powders begin to grow. Diamond particles with the diameter of 0.1 mm can be acquired under high temperature (1 070 K) that the ordinary graphite doesn't convert to diamond and the ordinary diamond crystal will convert to graphite. This phenomenon shows that the nanometric diamond particles can grow into larger diamond particles under lower temperature because of the surface activity of nanometric particles.