A preparative method has been proposed for obtaining chitosan products which have a desired degree of deacetylation (DD, %) without much decrease in molecular weight. Deacetylation was prepared by two methods. The method I which was proposed by Domard etc. was that DMSO/thiophenol was used repeatedly only during the deacetylation, the method II was that DMSO/thiophenol was used after the alkali treatment. Then, effective deacetylation was performed by the method II. To determine the degree of deacetylation, potentiometric titration method was adopted. According to the increase of DD, the viscosity average moleclar weght (Mv￣) and the DP of the chitosan products were decreased. The intensity of FT-IR peak appeared at 1655 cm^-1, which was characteristic peak of amide I (N -acetyl secondary amide group), was gradually decreased with deacetylation. X-ray analysis showed that the lattice-spacing in the chitosan film increased with the degree of deacetylation, but the difference of the transparency of the chitosan products was not diverged significantly.