[en] Effect of surrounding total gas pressure on oxidation rate of nuclear grade graphite with water vapor is an important factor in evaluating corrosion behaviors of graphite structural components of high temperature gas cooled reactor. The rate of oxidation with water vapor at high temperature is controlled by diffusion of gas through fine pore in graphite and by rate of chemical reaction. The oxidation rates of a nuclear grade graphite, SM1-24, with water vapor were measured at 1000degC within a range of 1 to 5 atm of helium. Diffusion of water vapor in pore was treated as counter diffusion of water/helium system in a uniform single capillary at a constant total gas pressure. An equation of oxidation rate under the condition of inpore diffusion control was derived as a function of wide range of total gas pressure. The calculated results were in good agreement with the experimental data of SM1-24 and also data of a graphite used in a literature. A flow parameter, β(p), is defined as a ratio of molecular diffusion resistance to Knudsen diffusion resistance of water vapor diffusion through pore and this value can be determined from the total gas pressure dependence of oxidation rate. Degree of contribution of the Knudsen diffusion to water vapor diffusion was evaluated on both SM1-24 and A3-27 graphites. (author)