Drastic effect of additional Mo on diamond nucleation was observed in the system of Mc60Cc40 (mixture of 60 mol% MgCO3 and 40 mol% CaCO3)-graphite at 7.7 GPa, 1700 and 1800 degrees C for 1 h. The decrease of temperature required for nucleation of diamond by addition of Mo is 200 degrees C, compared with the temperature in the system of Mc60Cc40-graphite (Sato et al., Diamond Related Mater. 8 (1999) 1900). These results indicate that a lower energy barrier is expected for the nucleation in the Mc60Cc40-Mo-graphite system. Therefore, this nucleation process in the system of Mc60Cc40-graphite-Mo can be considered to be different from the spontaneous nucleation in the system of Mc60Cc40-graphite. Two probable models for nucleation can be suggested: (1) heterogeneous nucleation in which nucleation is affected by MoC that is formed by the reaction between the carbonate and Mo and (2) achievement of enough concentration of carbon for formation of nuclei by the reaction between the carbonate and Mo.