Abstract: Conditions for acylation of chitosan (CTS) with maleic anhydride in DMF to give maleoyl chitasan (MLCTS) were improved, and the temperature and time of 90 ℃ and 9 h were chosen respectively for the reaction. The reaction product of MLCTS was studied by IRS and TG. It was shown in the IR spectrum that 2 absorption peaks of the conjugated crabonyl groups of maleic anhydride were disappeared, while stretching vibrational peak due to the C=O group of unsaturated carboxylic acid at 1 714.7 cm-1, and absorption peak due to duplicating of C=O of acetamido group and C=C of conjugated alkene at 1 650.3 cm-1 were observed. On the base of these facts, it was deduced that acylation of CTS by maleic anhydride was set giving the acylate MLCTS. It was shown by the results of IRS study, optimum acylation was attained by keeping the ratio of amount of substances (n) of CTS and maleic anhydride at 1 to 1.8. It was found by the results of TG analysis, that the thermostability of MLCTS was lower that CTS, which was deduced as a result of destruction of its crystalline structure due to introduction of C=O, C=C and OH groups into the CTS molecule.