Abstract: Polypropylene ( PP) was reinforced with nano-silica pret reated by graf ting polystyrene onto the particles.The microst ructure of the composites was studied by using atomic force microscopy ( AFM) and t ransmissionelect ron microscopy( TEM) . The observations were correlated with the tensile and impact properties of the composites. In order to illuminate the role of silica agglomerates in the composites , microscratching and nanoindentationwere investigated using AFM. The microhardness and dynamic mechanical properties (DMA) were examined as areference. It was found that the graf ted silica agglomerates possess load-bearing capacity , leading to differentresponses of the composites against the microhardness and DMA measurement s. The result s present direct evidencefor the effect of the t reated nano-silica agglomerates on the mechanical behavior of polypropylene. That is , the double percolation behavior of st ress volumet ric spheres accounted for the significant improvement on the mechanicalproperties of the composites.