Reactions between graphite and water were investigated at high pressures of 5.5 and 7.7 GPa and high temperatures from 1200 to 1500 °C for a duration of 24 h in a platinum sealed capsule. At temperatures above 1200 °C, at both pressures, graphite was recrystallized into well-crystallized flaky crystals. At 7.7 GPa, graphite partly transformed to diamond at 1400 °C and almost completely transformed at 1500 °C. At 5.5 GPa, no diamond with spontaneous nucleation was formed throughout the temperatures, but growth of diamond was observed on a diamond seed crystal at temperatures above 1300 °C. Fluids coexisting with solid carbon were analyzed by a mass spectrometer, and a small amount of CO2 was found to be present with H2O, showing that H2O-CO2 fluid was formed in the above HP-HT conditions by the dissolution of graphite into water.