The impact fracture mechanisms of polypropylene (PP), containing 9.2 vol % of calcium carbonate (CaCO3) nanoparticles, were investigated using optical microscopy and transmission electron microscopy. The incorporation of CaCO3 nanoparticles reduces the size of spherulites and induces the formation of P-phase crystallites, which leads to a more ductile PP matrix. Double-notch four-point bending (DN-4PB) Charpy impact specimens and notched Izod impact specimens were utilized to study the fracture mechanism(s) responsible for the observed toughening effect. A detailed investigation reveals that the CaCO3 nanoparticles act as stress concentrators to initiate massive crazes, followed by shear banding in PP matrix. These toughening mechanisms are responsible for the observed, improved impact strength. A comparison of the fracture mechanisms observed between DN-4PB Charpy and Izod impact tests is also made to show the effectiveness of DN4PB for investigation of impact fracture mechanisms of polymeric systems. (c) 2006 Wiley Periodicals, Inc.