LB film containing Acetylcholinesterase(AChE) was fabricated for the development of the fiber-optic biosensor to detect organophosphorus compounds in the contaminated water. The AChE-immobilized LB film was formed by adsorbing the enzyme molecules onto viologen monolayer using the electrostatic force. The optimal amount of the dissolved enzyme for the LB film formation were determined based on the enzyme adsorption isotherm and relative adsorption amount. The surface characteristics of AChE-immobilized LB film was analyzed by the atomic force microscope(AFM). The surface topography of the AChE-immobilized viologen LB films were obtained for the various conditions of the surface pressure. The response time to steady signal and the detection range of the sensor were 5 min and 0 similar to 2 ppm, respectively.