Mechanical properties and fracture toughness micromechanisms of copolypropylene filled with different amount of nanometric CaCO3 (5-15 wt %) were studied. J-integral fracture toughness was incorporated to measure the effect of incorporation of nanoparticle into PP matrix. Crack-tip damage zones and fracture Surfaces were studied to investigate the effect of nanofiller content on fracture toughness micromechanisms. It was found that nanofiller acted as a nucleating agent and decreased the spherulite size of polypropylene significantly. J-integral fracture toughness (J(c)) of nanocomposites was improved dramatically. The J(c) value increased LIP to approximately two times that of pure PP at 5 wt% of nano-CaCO3. The fracture micromechanisms varied from rubber particles cavitation and shear yielding in pure PP to simultaneous existence of rubber particles cavitation, shear yielding, filler particles debonding, and crazing in PP/CaCO3 nanocomposites. (C) 2008 Wiley Periodicals, Inc. J Appl Polym Sci 110: 4040-4048, 2008