Small diamond crystals less than 30 μm in size were crystallized from the starting system of graphite and oxalic acid dihydrate, (COOH)2·2H2O, at high pressure and high temperatures of 7.7 GPa and 1400-2000 °C, respectively. The crystals were euhedral with well-developed {1 1 1} faces similar to natural diamonds. Since it is believed that oxalic acid dihydrate first forms a C-O-H fluid mainly composed of H2O and CO2 by thermal decomposition, experiments suggest that graphite dissolved into the fluid and crystallized as diamond under the above high-pressure and high-temperature conditions.