Polypropylene (PP) impact copolymer is one of the heterophasic PP systems that is improved by rubber modification. Because the copolymer is a complicated polymer blend, which mainly consists of PP and ethylene-propylene rubber (EPR) components, the degradation behavior has hardly been studied. In this study, the thermal degradation of the copolymer was studied through direct observation by atomic force microscopy, which is a powerful tool for observing a local domain in a polymer blend. The degradation behavior was visually captured by the mapping of topological changes. Although the EPR phase was hardly degraded, the neighboring PP matrix was degraded selectively. The degradation behavior of the copolymer was found to be heterogeneous. (c) 2006 Wiley Periodicals, Inc.