Abstract: To modify the surface of nano-SiO₂, polytyrene and polyethylacrylate were grafted onto the particles through irradiation grafting polymerization, respectively. Mechanical testing of the modified nanosilica filled polypropylene (PP) composites reveals the significant strengthening and toughening effects of the treated nanoparticles on the matrix resin. It is believed that the extensive shear deformation of PP induced by the modified nanosilica is responsible for the improvement of the composites toughness. Microscopic observation further demonstrates that the modified nanoparticle aggregates become more compact due to the grafting polymer chains, which in turn provides an enhanced interfacial adhesion between the fillers and the matrix.