Formation of diamond was studied from the starting graphite and anhydrous oxalic acid (COOH)(2) in a platinum-sealed capsule at 7.7 GPa and 1300-1500 degrees C. Small octahedral diamond crystals < 10 mu m in size were crystallized and the fluid phase in coexistence with diamonds was found to be composed of H2O and CO2 by a mass spectrometer. Besides, when C-13 graphite was used as a starting material, (CO2)-C-13 was present with (CO2)-C-12 in the fluid, showing that a H2O-CO2 fluid was first formed by the decomposition of the oxalic acid, and then graphite dissolved into the fluid and precipitated as diamond.