[Objective] In order to study the relations among different positions,degrees of substitution and antioxidant ability.[Method] N,O-carboxymethyl chitosan(NOA,NOB and NOC)with various degrees of substitution(DS)were obtained by etherizing chito-oligosacchaside.Their structure and substituted degree were characterized and their antioxidant activity to ·OH was evaluated.[Result]The IC50s of NOA,NOB and NOC were 0.15,0.29,0.23 mg/ml while their DSs of-NH2 position(DSN)were 0.51,0.29 and 0.38 and DSO were 0.74,0.84,0.97respectively.[Conclusion]With the increase of DSN,antioxidant activity of N,O-carboxymethyl chitosan oligosaccharide to ·OH was up.