After surface modification of γ-Al2O3 nanoparticles with a silane coupling agent, styrene was graftcopolymerized onto γ-Al2O3 nanoparticles using a simultaneous γ-ray irradiation technique. The influence of the irradiation dose and the grafting kinetics were investigated in detail. Polypropylene nanocomposites were fabricated by blending polypropylene, polystyrene-grafted γ-Al2O3 (γ-Al2O3-g-PS), and a small amount of 1,4-butandiol dimethacrylate (1,4-BDDA) as a crosslinker, followed by e-beam irradiation. The polystyrene- grafted γ-Al2O3 and polypropylene nanocomposites were characterized by TGA, FT-IR spectroscopy, SEM, and UTM analyses. The polystyrene graft yield onto γ-Al2O3 increased with the absorbed dose. The graft yield was higher at 50 vol% of the styrene solution than at 70 vol%. The nanocomposite fabricated with 5phr of γ-Al2O3-g-PS and 3 phr of 1,4-BDDA showed the highest tensile strength. The homogeneous dispersion of γ-Al2O3-g-PS into the polypropylene matrix and the crosslinking through e-beam irradiation improved the mechanical properties of the nanocomposites.