Abstract: The structures and stabilities of carboxymethylchitosan and its derivatives were studied by using quantum chemical ab initio method. The calculated results indicated that the structure of carboxymethylchitosan in which the 2 site amino group of chitose(see Fig.2) was substituted by ethyloic was more stable than that in which 6 site hydroxyl group was substituted(see Fig.3). Between two isomers of carboxymethylchitosan deriva tives(i.e.(2hydroxyl3butoxyl）propylcarboxymethylchitosan, see Fig.4), isomer 1 was more stable than isomer 2. By using the optimized geometries, the vibration spectra of isomer 1 was calculated. And it was in good agreement with recent experimental results.

Key words: Carboxymethylchitosan, Chitosan derivative, （2-ydroxyl-3-utoxyl）propyl-arboxymethylchitosan, Quantum chemical ab initio method, Vibration spectra