The subject of this study was the crystallization behavior and thermal properties of polypropylene (PP)/maleic anhydride (MAH) modified nano calcium carbonate (nano-CaCO3) composites. In this study, 5 wt % nano-CaCO3 modified with different contents of MAH was filled into a PP matrix. X-ray diffraction and differential scanning calorimetry were used to characterize the crystal morphology and crystallization kinetics of a series of composites. The results demonstrate that the nano-CaCO3 modified with MAH had an important effect on the thermal and morphological properties of the nanocomposites. The Avrami exponent of the pure PP was an integer, but those of the composites were not integers, but the crystallization rate constant decreased as the content of MAH in the nano-CaCO3 filler increased in isothermal crystallization. In nonisothermal crystallization, the kinetic parameter F(T) and the degree of crystallinity of pure PP were compared with those of the PP composites filled with nano-CaCO3. We suggest that heterogeneous nucleation existed in the PP composites and that the transformation and retention of the beta-form crystal into the a-form crystal took place in the composite system and the b-form crystal had a higher nucleation rate and growth process than the a-form crystal in the PP composites. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 119: 1516-1527, 2011