Terminally hydroxylated isotactic polypropylenes (PP) with different molecular weights are grafted on SiO₂ nanoparticles (PP-g-SiO₂) to prepare PP/PP-g-SiO₂ nanocomposites. A significant increase of the elongation properties is obtained when the molecular weight of the grafted PP chains is sufficiently large. The origin of the improved elongation is attributed to the entanglement between the grafted and matrix chains.