Emulsion graft copolymerization of methylmethacrylate(MMA) onto gelatin in aqueous solution by potassium persulfate(KPS) with sodium lauryl sulfate(SLS) as an emulsifier was carried out under different conditions such as variation in the concentration of emulsifier, initiator, monomer, and gelatin, and in the reaction time and temperature. The molecular weight of grafted poly(methyl methacrylate) (g-PMMA) and the branching frequency were measured and thermogravimetric analysis of gelatin-g-PMMA was carried out. The obtained gelatin-g-PMMA was insoluble in water because of the high molecular weight. The molecular weight of g-PMMA increased with the increase in the reaction time and MMA concentration, but decreased with the increase in the KPS concentration and reaction temperature. The branching frequency of the grafted polymer was increased with the increase in the reaction time and temperature, and the concentration of KPS and MMA. Thermogravimetic analysis shows that 10% weight loss temperatures of gelatin-g-PMMA were increased with the grafted PMMA contents of the grafted polymer.