Oxidation-induced compressive strength damage of nuclear graphites was evaluated by the microstructure-related fracture model. Parameters used in the fracture model are grain size, pore size, pore size distribution etc. In the oxidation damage evaluation, growth of pores and change of fracture toughness due to the oxidation were considered in the model. Predicted results were compared with experimental data, and it was found that the prediction gives higher strength than the experimental data; this is thought to be the pore combination effect due to the oxidation.