Thermal oxidative degradation was carried out using four kinds of polypropylene impact copolymers, and the relationship between primary structure and degradation behavior was investigated by the results of C-13-NMR, TEM, GPC and the determination of hydroperoxide value (HPO) and number of chain scission (NCS). The degradation behavior mainly depended on the ethylene content and on the tacticity. The thermal degradation was found to be controllable by the primary structure.