Toughening mechanism of polypropylene (PP) blended with ethylene-propylene-diene rubber (EPDM) following selective crosslinking was examined in comparison with that of blends of PP before crosslinking. The yield stress, strength of craze, and density of void which are dominant factors for enhancing toughness in PP blends were evaluated and the deformation and fracture mechanism was discussed. It was concluded that toughness of PP blended with EPDM is improved by selective crosslinking, since the improvement of the craze strength is greater than the drop in the release of the constraint of strain.