SiO2 nanoparticles grafted to terminally hydroxylated polypropylene (PP-g-SiO2) with different molecular weights were melt mixed with PP to prepare a series of PP/PP-g-SiO2 nanocomposites. PP/PP-g-SiO2 offered several advantages over pristine PP and PP/unmodified SiO2 such as highly uniform dispersion up to 10 wt.-%, +200-400% faster crystallization and +30% increments for both the Young's modulus and the tensile strength without largely sacrificing the melt viscosity of PP. We concluded that grafted chains act as crystallization nuclei and co-crystallize with matrix chains to make PP-g-SiO2 nanoparticles as a physical cross-linker between lamellae, while the linkage disappears in melt and grafted chains minimize the cohesive attraction between nanoparticles. (C) 2014 Elsevier Ltd. All rights reserved.