In recent years, waste rubber in. the form of scrap tires has increased significantly in amount, and its disposal has become an important issue. We focused our attention on lipid peroxidation, which has a low environmental burden. Shredded vulcanized rubber samples were dipped in a small quantity of solvent, and stirred for 48 h. Significant reductions in molecular weights of the rubber samples were confirmed. From the structural analysis of the degradation product, formation of hydroxy and carbonyl groups, a decrease in beta-position methylene protons and olefin protons, and a reduction in carbon carbon double bonds were confirmed. Therefore, auto-oxidation is suggested as the reaction that reduced the molecular weight. The reaction mechanism involved lipid peroxidation by radical species that extracts hydrogen atoms. It is presumed that the polymer radicals generated undergo beta-scission reactions caused by auto-oxidation, resulting in reduction of the diene polymer molecular weight.