In recent decades, great attention has been devoted to the toughening of isotactic poly(propylene) (PP) with elastomers such as ethylene-propylene rubber (EPR). The most important reasons for this interest are the moderate cost and favorable properties of PP. This article is focused on the role of EPR in the deformation and fracture mechanism of PP/EPR blends with different volume fractions of elastomer phase. Differential scanning calorimetry (DSC), tensile tests, and microscopy techniques were used in this study. The fracture mechanism of isotactic PP toughened by EPR (PP/EPR) has also been studied by three point bending (3-PB) and four point bending (4-PB) tests. Rubber particle cavitation appears to be the main mechanism of microvoid formation, although some matrix/particle debonding was observed. The investigation of the toughening mechanism shows that a wide damage zone spreads in front of the pre-crack. Optical microscopy (OM) illustrates that, in pure PP, crazing is the only fracture mechanism, and no evidence of shear yielding is found, while in PP blends craze-like features associated with shear yielding are observed, which have been identified as high shear localized dilatational bands. This type of deformation pattern supports a model previously proposed by Lazzeri(1) to explain the interparticle distance effect on the basis of the stabilization effect on dilatational band propagation exerted by stretched rubber particles. (C) 2003 Wiley Periodicals, Inc.