By changing the amount of fluid source to that of graphite by the way of loading both materials, the effect of fluid concentration on the formation of diamond was investigated at 7.7 GPa and 1500 °C for 24 h in the system of graphite and CO2-H2O fluid, where anhydrous oxalic acid was used as a fluid source. In the samples where powders of oxalic acid and graphite were loaded separately into a platinum-sealed capsule, diamond yield was greatly affected by the fluid concentration. Namely, when the molar ratio of fluid to graphite was about 0.1, only a small amount of graphite of less than 20% was transformed into diamond, but the diamond yield increased to about 90% when the molar ratio of fluid increased to one. When powders of oxalic acid and graphite were mixed before being loaded, the transformation reaction became accelerated remarkably. In the samples where the molar ratio of fluid to graphite was about 0.6, for example, diamond yield reached 100% when a pre-mixed starting sample was used, while only about 40% of diamond was obtained without pre-mixing.