Small diamond crystals less than 30 mu m in size were crystallized from the starting system of graphite and oxalic acid dihydrate, (COOH), 2H(2)O, at high pressure and high temperatures of 7.7 GPa and 1400-2000 degrees 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.