Graft copolymerization of methlymethacrylate(MMA) onto sodium alginate(SA) aqueous solution by sodium metaperiodate(NaIO₄) as an initiator was carried out with the variation of the reaction time, the reaction temperature and the concentrations of initiator and monomer. The results obtained were as follows; 1. SA was easily separated from the reaction mixture of homopolymer, graft polymer and sodium alginate ungrafted by the treatment of concentrated boiling Na₃PO₄ solution with the small amount of Na₂SO₃ to the mixture and then isolated as the acid form by acidifying the salts solution containing SA. 2. The amount of graft polymer was larger than that of homopolymer below 70℃ whereas above 70℃ the amount of homopolymer was larger. 3. The sum of each amount of graft polymer and homopolymer was always increased with increase of the reaction time, the reaction temperature and the concentrations of initiator and monomer. 4. With increase of the concentration of initiator, the graft efficiency was increased below 70℃ while decreased above 70℃. 5. Graft copolymerization of MMA onto SA in aqueous solution was carried out without initiator.