AbstractThe 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 iPP, 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. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 1889–1898, 2006