An attempt was made to prepared chitosan antibacterial pro-drug that is suitable for treatment agent of fish disease. N-methyleneylsulfapyrazine of chitosan and chemical modification of the derivative have been examined. Chloroacetylaldehyde was reacted with sulfadiazine derivative which has amine bond, to give a coupled product of chitosan-N-methyleneylsulfadiazine derivatives[2,3,4]. The reaction mechanism for the present coupling method will be presented in detail.