Carboxymethyl chitosan(CMCh) was prepared by chemical reaction with mono-chloroacetic acid to low molecular weight water soluble chitosan(LMWSC) of C6-OH group to apply as gene and drug carrier. O-Carboxymethylchitosan(O-CMCh) is a kind of biocompatible derivatives of chitosan whose water solubility is strongly dependent on the degree of carboxymethylation. The carboxymethylation of a chitosan of molecular mass (20 kDa) and deacetylation degree (96%) introduced 0.98 O-Carboxymethyl groups per repeating unit. The structure were characterized by 1H-NMR. 13C-NMR, DEPT.