The gamma-radiation-induced graft copolymerization of 2,2,6,6-tetramethyl-4-piperidinyl methacrylate (TMPM) onto polypropylene (PP) was investigated using a simultaneous-irradiation technique. The effects of solvents, atmospheres, dose, and monomer concentration on the percent of grafting were studied. The grafted TMPM showed better thermal stability, solvent extraction resistance, and compatibility with PP than monomeric TMPM or polymeric PTMPM. The stabilizing effectiveness of grafted TMPM on the radiation resistance of PP was found to be better than that of monomeric TMPM. The combination of grafted TMPM and low molecular weight hindered amine fight stabilizers showed higher stabilizing effectiveness.