Using the porous polypropylene(PP) granules consisting of submicroparticles prepared by the reactor granular technology(RGT), a facile process was developed to synthesize in situ polypropylene/silica nanocomposites where styrenealt-maleic anhydride copolymers(PSM) were firstly grafted onto both the outer- and the inner-pore surfaces of PP granules via solid-phase grafting polymerization, and then a sol-gel reaction was conducted inside these micro-pores with the carboxyl acid derived from maleic anhydride as catalyst. TEM micrographs demonstrated that the silica nanoparticles obtained by this strategy had a size around 10?20 nm in diameter and uniformly dispersed in the PSM-based nanocomposites. More importantly, the silica nanoparticles could significantly reduce the size of PP spherulites. Consequently, the PP/silica nanocomposites with high transparency and good mechanical performance were achieved. Using the porous polypropylene (PP) granules consisting of submicroparticles prepared by the reactor granular technology (RGT), a facile process was developed to synthesize in situ polypropylene / silica nanocomposites where styrenealt-maleic anhydride copolymers (PSM) were first grafted onto both the outer - and the inner-pore surfaces of PP granules via solid-phase grafting polymerization, and then a sol-gel reaction were conducted inside these micro-pores with the carboxyl acid derived from maleic anhydride as catalyst. by this strategy had a size around 10 ~ 20 nm in diameter and uniformly dispersed in the PSM-based nanocomposites. More importantly, the silica nanoparticles could significantly reduce the size of PP spherulites. Consequently, the PP / silica nanocomposites with high transparency and good mechanical performance were achieved.