The crystallization behavior of polypropylene (PP) copolymer obtained by in situ reactor copolymerization with or without a nucleating agent and/or nano-CaCO3 particles was investigated both by thermal analysis and by polarized light microscopy. The Avrami model is successfully used to describe the crystallization kinetics of the studied copolymer. The results of the investigation show that a dramatic decrease of the half-time of crystallization t(1/2) as well as a significant increase of the overall crystallization rate, are observed in the presence of the nucleating agent. These effects are further promoted in the presence of the nano-CaCO3 particles. The incorporation of the nucleating agent and nano-CaCO3 particles into PP copolymer remarkably improved the mechanical properties and heat distortion temperature. (C) 2003 Wiley Periodicals, Inc. J Appl Polyrn Sci 91: 431-438, 2004.