N-butyrylated chitosan with various degree of N-butyrylation (DB) were prepared by mixing completely deacetylated chitosan and butyric anhydride in mild condition without serious degradation and O-substitution. A standard curve to measure DB was obtained by plotting the infrared absorbance ratio of A(1560)/A(2880) against the known DB (from 0%similar to 99%) of eight specimens. The influence of DB on the critical concentration C* of N-butyrylated chitosan/dichloroacetic acid solutions required to form mesophase was investigated by optical method. A maximum C* value appeared at medium DB. The effect was explained by the disordering of chain with medium composition ratio of the copolymer of glucosamine and N-butyryl glucosamine.