AbstractThe degradation of dicumyl peroxide‐cured natural rubber was studied by the stress relaxation method. Experiments were carried out in air, in nitrogen, and by irradiation of UV light in the temperature range of 30–200°C. We attempted to divide factors of the degradation into two parts: that caused by heat and that by light, and to study the respective degradation quantitatively. The degradation caused by UV light in nitrogen was almost independent of temperature. However, the degradation caused by UV light in air increased with temperature and the rate of degradation caused by the interaction between light and oxygen was greater at high temperature than at low temperature. It was found that scission along main chains occurs for dicumyl peroxide‐cured natural rubber by photo‐oxidation as well as thermal o