Abstract： Chitosan and its derivatives are unique cationic copolymer of glucosamine and Nacetylglucosamine. They have drawn much attention in various fields, including pharmaceutical, biomedicine, water treatment, cosmetics, agriculture, and food industry, due to their useful characteristics such as biorenewabilty, biodegradability, biocompatibility, bioadhesivity and nontoxicity. This paper introduced the synthetic method of different molecular weight, different degree of deacetylation and Omodified chitosan. Moreover, it revewed the recent free radical scavenging activities of chitosan and its derivatives and discussed in brief the roles molecular weight, degree of deacetylation and Omodification played in free radical scavenging.