A convenient method to synthesize starch/lactic acid copolymer was studied in this work. The lactic acid grafting starch copolymer was directly prepared by reaction of cornstarch with lactic acid catalyzed with stannous chloride (SnCl2). The structure of starch/lactic acid copolymer was characterized by IR and 1H-NMR, and its morphology was seen by SEM and XRD. The effects of mass ratio between starch and lactic lacid, amount of catalyst and reaction time on the grafting degree were also studied. The result showed that the best grafting degree could reach 12.8％ with 1:6 as the mass ratio between starch and lactic acid and 1.5％ as amount of catalyst accounting for consumption of lactic acid by reaction at 90 oC for 7 h.