Influences of polypropylene (PP) grafted to SiO2 nanoparticles (7 nm) were studied on the crystallization behavior and the mechanical properties of PP/SiO2 nanocomposites. PP for the matrix and grafting was synthesized in order to have an identical primary structure, aiming at their co-crystallization and resulting reinforcement of filler-matrix interfaces. The grafted PP chains improved the dispersion of SiO2, and notably accelerated nucleation in crystallization. It was plausible that the grafted chains whose one chain end was pinned to SiO2 became nuclei of the crystallization (co-crystallization between the matrix and grafted chains), thus directly bridging between the matrix and SiO2 nanoparticles. The Young's modulus and tensile strength were most improved by the grafted PP chains at low filler contents such as 2.3 wt%, whose origin was attributed to effective load transfer to SiO2 through the co-crystallization-mediated bridging.