Isothermal and non-isothermal crystallization kinetics of polypropylene (PP)/surface-treated SiO2 nanocomposites were extensively studied. Analysis of the isothermal crystallization showed that the phenomenon is characterized by faster rates as the amount of silica is increased. In the case of non-isothermal crystallization, it was found that the Ozawa analysis was rather inapplicable for the nanocomposites. In contrast, the modified Avrami method, as well as the method proposed by Mo was applied giving satisfactory results. The effective energy barrier for non-isothermal crystallization was estimated as a function of the relative degree of crystallinity using the isoconversional analysis of calorimetric data. This was found to vary with the degree of conversion, as well as with the presence of filler. Finally, the nucleation activity of the silica nanoparticles on the polymer matrix was explored and it was proved that when the content of filler exceeds 7.5 wt.% the nucleation is not drastically improved. (C) 2004 Elsevier B.V. All rights reserved.