An irradiation grafting method was applied for the modification of nanoparticles so that the latter can be added to polymeric materials for improving their mechanical performance using existing compounding techniques. The following items are discussed in this paper: (a) chemical interaction between the grafting monomers and the nanoparticles during irradiation, (b) properties including modulus, yield strength, impact strength and fracture toughness of the resultant composites, and (c) possible morphological changes induced by the addition of nanoparticles. Through irradiation grafting polymerization, nanoparticle agglomerates turn into a nano-composite microstructure (comprised of the nanoparticles and the grafted, homopolymerized secondary polymer), which in turn builds up a strong interfacial interaction with the surrounding,primary polymeric matrix during the subsequent mixing procedure. Due to the fact that different grafting polymers brought about different nanoparticle/matrix interfacial features, microstructures and properties of the ultimate composites could thus be tailored. It was found that the reinforcing and toughening effects of the nanoparticles on the polymer matrix can be fully brought into play at a rather low filler loading in comparison to conventional particulate filled composites.