The aim of this study was to compare the effects of different silica grades on the structure and morphology of isotactic polypropylene (iPP)/silica composites to better understand their structureproperty relationships. Isotactic polypropylene composites with 2, 4, 6, 8 vol % of added silica fillers differing in particle size (micro- vs. nanosilica) and surface modification (untreated vs. treated surface) were prepared by nonisothermal compression molding and characterized by different methods. The addition of all silica fillers grades to the iPP matrix significantly influenced the spherulitic morphology, while phase characteristics of the iPP matrix seemed to be unaffected. Surface modification of silica fillers exhibited stronger effects on spherulite size than size of silica particles. Nonpolar silica particles, more miscible or compatible with iPP chains than polar silica particles, enabled better spherulitic growth. The spherulite sizes tended to reach equal values at 8 vol % of added silicas showing that spherulite size became independent of filler concentration and surface modification above optimum filler concentration. (c) 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013