Graft copolymerization of acrylic acid(AA) and styrene onto poly(ethylene terephthalate)[PET] film using benzoyl peroxide[BPO] as an initiator was investigated under the various conditions. The effect of the monomer concentration, of the initiator concentration, and of the grafting temperature and time on the graft yield was evaluated. It was found that the maximum graft yield was obtained when 80/20(mole ratio) styrene/AA mixture was used. Increasing the ratio of styrene in the styrene/AA mixture enhanced grafting. It was studied that there is a synergistic phenomenon when styrene/AA mixtures are grafted to PET film. The results indicated that, in the styrene/AA grafting, styrene exhibits negative synergistic effect and AA exhibits positive one. Grafting of AA and styrene to PET film improves the moisture regain of the film considerably. Moisture regain of PET-g-styrene is higher than that of PET-g-AA.