Chemiluminescence analysis was applied to evaluate oxidation of ethylene propylene diene copolymers (EPDMs), and the results were analyzed through comparisons with those of low density polyethylene (LDPE) and cross-linked polyethylene (XLPE) with or without antioxidant. If LDPE or XLPE is being oxidized, CL appears abruptly in a frequency range from 300 to 850 nm after a certain incubation time. This incubation time corresponds to oxidation induction time (OIT). In contrast to LDPE or XLPE, when EPDM samples are heated in air, a strong CL intensity is observable from its initial stage, even if the amount of antioxidant in the sample is high enough. However, after the CL intensity decays once, it increases gradually to a certain peak and then it decays again. We can regard the time at which CL shows the peak as OIT, because it shows a positive correlation with the weight content of antioxidant in the sample. Furthermore, after the EPDM samples were aged in different conditions, namely by heat alone, by gamma-rays alone, and by simultaneously given heat and gamma-rays, the dependence of OIT on the aging period was examined for each condition. The resultant dependence of OIT was found to agree with the dependence observed by measuring the elongation-at-break and also with the one measured by differential scanning calorimetry.