Abstract: Nuclear grade graphite is widely used as structure material, moderating material and reflector material in the high temperature gas-cooled reactor. The oxidation performance of nuclear grade graphite is important to the reactor material corrosion during water or air ingress accidents. The initial porosity distribution and porosity change during the oxidation process have an influence on the oxidation performance. The nuclear grade graphite IG-110, H-451, NBG-18 and A3-3 were studied in this paper. Based on the one dimensional transient oxidation model, the oxidation of spherical nuclear grade graphite with a diameter of 6 cm was researched. The effects of different initial porosity distributions on the graphite oxidation were analyzed as the initial porosity distribution was uniform, normal and lognormal respectively. For conservative consideration and simplifying the oxidation model in the safety analysis of high temperature gas-cooled reactor, the initial porosity distribution of nuclear grade graphite should be selected as uniform, which results a maximum overall weight loss ratio of graphite.