Abstract: The effective diffusion coefficient is an important parameter describing the diffusive mass transfer inside graphite. The single average pore size assumption can not reflect the complex pore size distribution and mass transfer in graphite, and the existing formulas for calculating effective diffusion coefficient of nuclear graphite differ greatly from experimental results. Considering the influence of Knudsen diffusion, the pore size range of nuclear graphite was divided into two categories according to the micropore size distribution characteristics and diffusion theory, and the formula of effective diffusion coefficient was modified. Then the pore size analyses of unoxidized and isothermal oxidized IG110 nuclear graphite samples at different temperatures were carried out by mercury intrusion meter. The effective diffusion coefficient was calculated using the modified formula and compared with experimental results. The influence of weight loss rate, temperature and pressure on the effective diffusion coefficient was discussed. The calculated results using the modified effective diffusion coefficient formula are consistent with the experimental results. The calculation accuracy is improved. The effective diffusion coefficient increases with the weight loss rate, and increases more significantly when the weight loss rate is lower than 20%. The effective diffusion coefficient is proportional to 0.528 power of temperature, while it is proportional to approximately -0.33 power of pressure.