The formation process of diamond from supercritical H2O-CO2 fluid was studied using 13C-graphitic carbon and oxalic acid dihydrate, (COOH)2·2H2O, as starting materials under a diamond stable high pressure-high temperature (HP-HT) condition of 7.7 GPa and 1600 °C. The exchange reaction between 13C-graphitic carbon and 12CO2 in the supercritical H2O-CO2 fluid, which was first formed by the decomposition of oxalic acid dihydrate, occurred very rapidly and became nearly equilibrated after 6 h. At the same time, graphite was recrystallized and coexistent with the fluid until traces of diamond were first observed after 8 h. All graphite transformed into diamond after 17 h, showing that a considerably long induction time was present for the formation of diamond in this fluid system.