In this study, the thermal oxidative degradation of a polypropylene (PP) impact copolymer was examined using a transmission electron microscope equipped with an energy dispersive X-ray system (TEM/EDX). The oxidative behavior was visually captured by oxygen line analysis. The oxidation resistance of the ethylene-propylene rubber (EPR) phase was considerably higher than that of the PP matrix, indicating that the degradation behavior of the PP impact copolymer was heterogeneous. It was found that the higher resistance of the EPR phase originated from its primary structure by C-13-NMR measurement.