Oxidation velocities of isotropic graphite, pyrolytic carbon, silicon and boron mixed graphites were measured by using a vacuum microbalance of Cahn RG type under an oxygen pressure of 1.33 kPa and temperature range of 650~900°C. For isotropic graphites with different ash concentrations, it was observed that the oxidation velocity increased as the ash concentration when the temperature was below 750°C. The pyrolytic carbon had the oxidation velocity approximately an order of magnitude smaller than that of the isotropic graphite. The oxidation velocity of silicon mixed graphite linearly decreased as the increase of silicon concentration. In a case of boron mixed graphite, the oxidation was considerably suppressed by the boron content and the weight gain was observed due to the formation of B2O3 when the boron concentration exceeded approximately 10%.