We demonstrate that deposition of gas barrier film is effective to suppress radiation oxidation of a polymer and can improve its radiation resistance. The gas barrier silica films were successfully formed on polypropylene by magnetron sputtering. Long-term irradiation of cobalt-60 gamma-rays in air resulted in oxidation of samples without silica coating, whereas samples with silica coating were hardly oxidized. Furthermore, the radiation resistance of polypropylene was significantly improved by silica coating. Because of non-toxicity of silica, the technique may be applicable to polymeric medical items subjected to radiation sterilization.