Nuclear graphite has served as neutron moderator, reflector and structural material in various nuclear reactors due to its excellent irradiation performance and mechanical properties at high temperature. However, the microstructure of nuclear graphite contains over 10% porosity by volume. During the lifetime of reactors, the porosity of nuclear graphite will be increased due to the irradiation-induced volume changes and/or radiolytic oxidation. Consequently, it is important to predict the effect of porosity on the mechanical properties of nuclear graphite. In this paper, a novel multi-scale method was used to study the effect of porosity on Young's modulus. Numerical results suggested that Young's modulus versus porosity can be predicted by the Knudsen law. Then, the influence of different pore distributions on Young's modulus was analysed. Finally, Young's modulus with different pore sizes and the effect of pore shape were studied. And a relationship between decay coefficients in the Knudsen law and the pore shape ratios was found.