Chitosans with different average molecular weight (1 500-156 kDa) were prepared under optimal conditions based on our previous research by the depolymerization of initial chitosan using hydrogen peroxide,with maintaining the native structure of natural chitosan as principal consideration.Nitrogen and Carbon components of initial chitosan and degraded products were analyzed by BCHI fully automatic nitrogen determination system and Liquid total organic carbon (TOC) analyzer respectively.Fourier transform infrared (FTIR),13 C nuclear magnetic resonance (13 C NMR),X-ray diffraction and circular dichroism (CD) analyses were used to characterize the structure properties of samples.The results indicated that the maximum standard deviations of carbon content and nitrogen content and the degree of deacetylation (DD) of degraded products with the corresponding values of the initial chitosan are 2.4%,2.3% and 6.9% respectively.Besides,the differentiations of the nitrogen content and DD value of the degraded product with the corresponding values of the initial chitosan are narrowed with the increase in the reaction time.FTIR spectra of resulting products are similar to that of initial chitosan in terms of peak number and position,indicating that there are no other functional groups formed during the degradation.13 C-NMR analyses of initial chitosan and degraded products revealed that the chemical structures of resulting chitosans are not changed to any noticeable extent.X-ray diffraction patterns of initial chitosan and degraded chitosans are alike and show characteristic peaks at 2θ=10.4°and 19.8°under the condition that the initial chitosan was disposed as degraded chitosans.Circular dichroism analyses showed that all the samples exhibit a broad negative band located at about 210 nm assigned to n→π electronic transition of the —NH—CO— chromophore on a glycosidic ring in acidic media,which demonstrated that degraded chitosans maintain their natural conformation in liquid state substantially.All these confirmed that the degraded chtiosans maintain their natural structure and conformation,and the breakage of β-1,4-glucoside bonds in macromolecule is the basic process under optimal degradation conditions.