The effect of reactive compatibilization on the mechanical properties of nanosilica filled polypropylene (PP) composites was studied in this work. First, the nanoparticles were grafted with poly(glycidyl methacrylate) (PGMA) by solution free-radical polymerization, and then melt blended together with PIP matrix and aminated PP (PP-g-NH2) that acts as reactive compatibilizer. The reaction between epoxide groups of the grafted PGMA on the nanoparticles and amine groups of PP-g-NH2 during compounding greatly improved interfacial interaction in the composites. As a result, tensile strength, Young's modulus, and notch impact strength of PP composites were increased at rather low filler content. The experimental results indicated that the reinforcing and toughening effects were controlled by flexibility of the grafted polymer as well as processing methods.