Abstract: Nuclear grade graphite IG-110 is widely used as structure materials in the high temperature gas-cooled reactor pebble-bed module (HTR-PM). Under normal operation condition, or air/water ingress accidents, nuclear grade graphite IG-110 is inevitably oxidized by oxygen and steam. The oxidation performance of IG-110 is important to the reactor safe operation. A new oxidation model, considering the local porosity and oxidation rate varying with weight loss ratio, was developed. Based on the oxidation model, the parameters trending over time and space could be obtained. The effect of porosity and Knudsen diffusion on the effective diffusion coefficient was mainly analyzed. The appropriate effective diffusion coefficient for the safety analysis could be determined based on the numerical results.