Experiments on diamond crystallization in kimberlite melt were performed for 40. h at 6.3. GPa in the temperature range of 1300-1570. °C and at 7.5. GPa in the temperature range of 1450-1570. °C, using a multianvil high-pressure apparatus of split-sphere type. Group I kimberlite from the Udachnaya-East pipe and a synthetic multicomponent mixture modeling the average composition of group II kimberlites were used as starting materials. The experiments have shown that diamond growth on seed crystals in the kimberlite melt in equilibrium with olivine, pyroxene, and garnet starts from 1400. °C at 7.5. GPa and from 1520. °C at 6.3. GPa. Diamond nucleation requires higher temperature and pressure, 1570. °C and 7.5. GPa. The alkali-enriched and silicate-depleted derivates of kimberlite melts ensure the growth and nucleation of diamond at lower P and T values: 1400. °C at 7.5. GPa and 1520. °C at 6.3. GPa. The results obtained evidence that temperature, pressure, and the composition of crystallization medium are the main factors controlling diamond formation processes in the kimberlite melts and their derivates.