Abstract: The tensile strength and the area under the tensile stress-strain curve of irradiation grafted and nano-silica filled polypropylene are studied to understand the interfacial effects and the percolation behavior induced by the nanoparticles. It is found that the nanoparticle agglomerates has a microstructure consisting of nanoparticles and grafted polymer after irradiation grafting polymerization, and thus a strong interfacial interaction can be established easily between the nanoparticles and matrix. In addition, an effect of double percolation of stress volumes, which might be responsible for the improvement of the tensile properties of the nanocomposites, is discussed in detail.