A kinetic model that combines gamma irradiation, chemical reactions, gas diffusion, and molecular scission is developed to investigate the oxidation process of ethylene-propylene-diene (EPDM) copolymer. The chemical reactions such as free radical generation, oxidation, and recombination in the model are solved by an implicit scheme. The diffusion equations of gaeous oxygen, hydrogen, and methane are numerically solved by a stable and highly accurate method. The time dependent concentrations of free radicals, oxidation products, and gases as a function of position in EPDM are obtained from the simulation. The results show that the degradation of EPDM is limited by oxygen diffusion. In addition, the degradation is severer near the surface than inside, which is consistent with an experimental result on the surface hardness of EPDM observed by scanning probe microscopy.