Abstract: There are huge amounts of nuclear graphite serving as structural material in high temperature gas-cooled reactors (HTGRs). It is essential to study the long-term corrosion behavior of the graphite materials in core. A reasonable oxidation model considering the effects of porosity changing and the burn-off was established. Using the model, the transient oxidation behaviors of a semi-infinite graphite slab and a finite graphite tube were analyzed. It is found the oxidation depths decrease with the temperature increasing. The simulation results were also compared with the experimental results and calculation results in literature to verify the effectiveness of the oxidation model.