Inorganic nanofillers, CaCO3 and nanoclay, are widely applied to improve the mechanical properties of polypropylene (PP). In general, the use of spherical CaCO3 can enhance the impact strength while the use of layered nanoclay can enhance the modulus and yield stress. With the objective to simultaneously improve the stiffness, strength, and impact strength of PP, in this work a ternary nanocomposite, PP/CaCO3/nanoclay (NCPP), was prepared and its morphology, crystallization, and mechanical behaviors were investigated with a comparison to the binary nanocomposites, PP/CaCO3 (CPP) and PP/nanoclay (NPP). The results showed that in NCPP the nanoclay was extremely exfoliated with a much higher degree than that in NPP, which was possibly because the incorporation of CaCO3 nanoparticles adjusted the matrix viscosity and thus provided a balance between shear stress and molecule diffusion. As a result of this highly exfoliated structure, a substantial increase in modulus and yield stress was attained in NCPP. However, its impact strength was less enhanced. The toughening effects of the CaCO3 particles observed in CPP became ineffective. This difference was ascribed to the fact that in NCPP the crystallization behavior was dominated by the nanoclay and the formation of beta-phase crystallites induced by the CaCO3 particles was inhibited. (c) 2007 Wiley Periodicals, Inc.