Thermogravimetric analysis (TGA) has been used for obtaining activation energy which is necessary for deciding the condition of short time of thermal aging. The activation energy obtained by TGA and that obtained by thermal aging at constant temperature was compared in this article. The sample was tetrafluoroethylene-propylene elastomer (TPE) whose thickness was 0.15 mm. The rate of mass loss at constant temperature increased from oxidation induction time (OIT). Activation energy obtained by TGA was higher than that obtained at constant temperature and the value depended on the rate of temperature rise. The pre-irradiation of gamma ray of 500kGy disappeared OIT observed at constant temperature. The pre-irradiation decreased the activation energy obtained by TGA. The value was from 143 kJ/mol to 157 kJ/mol, depending on the given conversion. Table 1 shows the summarized result. Table 1 Activation energy of TPE obtained with various view points Focused point Activation energy /kJ mol-1 The conversion on constant temperature1) 110-123 OIT observed on constant temperature 106 TGA, higher rate of temperature rise 219-237 TGA, lower rate of temperature rise 153-173 TGA, weight started to decrease 186 TGA, inflection point (Second derivative is 0) 170 1) The given conversion was 90%, 80%, 70%, 60% and 60%.