Impact polypropylene copolymer (IPC) has been known as a multiphase material in which an ethylene-propylene (EP) random copolymer, serves as toughening component, is dispersed in the homo-polypropylene hPP matrix. The crystalline EP copolymer (cEP) is another component whose role and microstructural effect on the IPC properties has not been well understood. This work reveals the relationship between the microstructure of cEP and the mechanical properties, that is, impact and tensile resistance, of IPC. We clarify that IPC comprising high contents of cEP with long homo-PP segment can extend the elongation at break while cEP with high content of homo-PE segment contributes to high impact strength. Mechanisms for both of these processes have been proposed. (c) 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013