Acetylation of chitin broke the intermolecular hydrogen bond between -6OH group and amide group, and enhanced solubility of chitin to organic acid. The acetylchitin was dissolved in formic acid. The solution was cast on the glass after filtration. Film was prepared by two method. The one was that the cast solution was coagulated in non-solvent such as acetone. The second was prepared by drying the cast solution in the hood so that the dense film was formed. Degree of substitution was determined by both Kjeldahl method and infrared absorption spectra. The intensity of peak appeared at 1620cm-1, which is characteristic peak of a-chitin, was gradually decreased with acetylation. This fact was attributed to breaking of hydrogen bond between -6OH group and amide group. In the mechanical properties of acetylchitin film, the tensile strength of air-dry film was better than that of coagulated film. Tensile strength was increased with substitution.