Toughening mechanisms and mechanical properties of two high-crystallinity polypropylene (hcPP)-based composite systems, hcPP/talc and hcPP/CaCO3, are investigated. Significant improvement in tensile modulus is observed in the PP/talc composite, but only a moderate improvement is found for hcPP/CaCO3. The introduction of CaCO3 nanoparticles to hcPP helps nucleate a measurable amount of beta-phase crystals and results in a significant drop in crystallization temperature, suggesting a possible retardation of hcPP crystallization. In addition, the hcPP/CaCO3 nanocomposite shows more pronounced damping characteristics than that of hcPP/talc, throughout the temperature range studied. A detailed investigation of fracture mechanisms suggests that well-dispersed, highly oriented talc particles cause embrittlement of hcPR Only when the crack extends toward the edges of the specimen will the crack deflection/bifurcation and microcracking mechanisms initiate. In the case of hcPP/CaCO3, the CaCO3 nanoparticles help trigger massive crazing and shear yielding if the testing speed is in quasi-static. The presence of beta-phase crystals around the CaCO3 particles could facilitate the formation of crazes throughout the hcPP matrix. Approaches for toughening hcPP are discussed. (c) 2006 Springer Science + Business Media, Inc.