This article reports a comparative study of polypropylene (PP) nanocomposites synthesized with nanosilica (NS) and diglycidyl ether of bisphenol A, an epoxy-resin-grafted nanosilica (ENS), as nanofillers. These nanocomposites were prepared with the melt-mixing method at a constant loading level of 2.5 wt %; this loading level was much lower than that used for fillers in conventional composites. The effects of pure NS and ENS on the thermal, structural, mechanical, and dynamic mechanical properties of PP were analyzed with wide-angle X-ray diffraction, transmission electron microscopy, thermogravimetric analysis, differential scanning calorimetry, dynamic mechanical analysis, and scanning electron microscopy. The transmission electron microscopy studies showed a better dispersion of ENS in the PP matrix, that is, in the polypropylene-epoxy-resin-grafted nanosilica (PP-ENS) nanocomposite, in comparison with NS in the PP matrix, that is, in the polypropylene-nanosilica (PP-NS) nanocomposite. Also, the thermogravimetric analysis results showed a higher thermal stability for PP-ENS than PP-NS. Furthermore, the dynamic mechanical analysis studies showed an increase in the elastic modulus and glass-transition temperature for PP-ENS with respect to PP-NS. (c) 2006 Wiley Periodicals, Inc.