Using the radiation peroxide grafting technique, a low density polyethylene film was graft copolymerized with acrylic acid and methacrylic acid in the presence of additives such as acid and metallic salts. The effect of acid and metallic salts on the grafting yield and reaction mechanisms were examined. The results showed that the inclusion of an appropriate cationic salt and acid in acrylic acid and methacrylic acid grafting solutions was extremely beneficial and led to a most unusual enhancement effect in the radiation grafting. On the other hand, inclusion of acid in the grafting solution in the absence of cationic salts led to a negative effect on the grafting reaction. It is considered that sulfuric acid accelerates the decomposition of hydroperoxides in the presence of a metallic salt such as Fe2+ to form radicals that can initiate a grafting reaction. Of the metallic salts for inhibiting homopolymerization, only oxidized metallic salts produced the high grafting yield of acrylic acid and methacrylic acid.