lsotactic polypropylene (PP) and calcium carbonate (CaCO3) nanocomposites were prepared by melt extrusion in a twin screw extruder. The commercial CaCO3 nanoparticles had a poor dispersion in PP matrix. The addition of a small amount of a nonionic modifier during melt extrusion greatly improved the dispersion of CaCO3 nanoparticles. The influence of CaCO3 nanoparticles on the crystallization of PP was studied by wide angle X-ray diffraction and polarized optical microscopy. The introduction of CaCO3 particles resulted in small and imperfect PP spherulites, decreased spherulite growth rate and induced formation of P-form PP. The yield strength of PP decreased gradually while its Young's modulus increased slightly with increasing CaCO3 loading. By adding 1.5 wt% of nonionic modifier to PP/CaCO3 (85/15) nanocomposite these tensile properties were not changed much but the notched Izod impact energy of the composites was significantly increased. (C) 2004 Elsevier Ltd. All rights reserved.