Mechanism of graft copolymerization of vinyl monomers onto chitosan initiated by Ce(IV) ion, one of the important ways of chemical modification of chitosan, has been investigated by means of kinetics measurement and polymer chain structure analysis. It is found that when Ce(IV) ions reacts with adjacent hydroxylamine structure in chitosan, a chelate complex is first formed and then it disproportionates to radical for initiation of graft copolymerization. There exist two ways for such initiation depending upon the reaction temperature. For a reaction temperature lower than 40 degrees C, a -CHO group and a -C = NH group are introduced, and the aldehyde group reacts with Ce(IV) ions to form an acyl radical which initiates a graft polymer chain in this saccharide unit. For temperatures higher than 90 degrees C, the -C = NH group hydrolyzes to form an amine and an aldehyde group which also reacts with Ce(IV) ions. In this case there should be two aldehyde groups, i.e. two initiation sites, in one adjacent hydroxyl-amine structure. That means that the initial radical in the chitosan/Ce(IV) system is similar to that in the cellulose/Ce(IV) one, but the former usually gives a lower grafting reactivity than the latter due to the higher stability of the chelate.