The crystallization and melting behavior of isotactic polypropylene (iPP) and polypropylene copolymer (co-PP) containing silicon dioxide (SiO2) were investigated by differential scanning calorimeter (DSC). SiO2 had a heterogenous nucleating effect on HIT, leading to a moderate increase in the crystallization temperature and a decrease in the half crystallization time. However, SiO2 decreased the crystallization temperature and prolonged the half crystallization time of co-PP. A modified Avrami theory was successfully used to well describe the early stages of nonisothermal crystallization of iPP, co-PP, and their composites. SiO2 exhibited high nucleation activity for iPP, but showed little nucleation activity for co-PP and even restrained nucleation. The iPP/SiO2 composite had higher activation energy of crystal growth than iPP, indicating the difficulty of crystal growth of the composite. The co-PP/SiO2 composite had lower activation energy than co-PP, indicating the ease of crystal growth of the composite. Crystallization rates of iPP, co-PP, and their composites depended on the nucleation. Because of its high rate of nucleation, the iPP/SiO2 composite had higher crystallization rate than iPP. Because of its low rate of nucleation, the co-PP/SiO2 composite had lower crystallization rate than co-PP. (c) 2006 Wiley Periodicals, Inc.