To introduce the legands which bind La(III), the carboxylic group containing monomers, such as acrylic acid (AA) and methacrylic acid(MA) were graft copolymerized onto PET film using benzoyl peroxide as an initiator, The two monomers show synergistic effect on the graft yield. The maximum graft yield could be obtained at the monomer ratio (mole ratio) of 1 : 4(AA : MA). But the consideration of the amount of the copolymers not grafted and the ease of the binding with La (III), the ratio of 1 : 1, which gives considerable synergistic effect, is suitable for introducing carboxylic groups onto PET. The conversion of the grafted copolymers to sodium salt with sodium carbonate was studied. The grafted PET is hydrolyzed easily in weak alkaline solution because the grafted hydrophilic copolymers make the penetration of alkaline ion easy by the swelling of grafted PET in water. So to convert the carboxylic group into sodium salt the weak alkaline solution of low concentration should be used. It seems appropriate to use 0.05% sodium carbonate solution.