The surface-plasma-wave technique is a very sensitive way used for determining the optical constants of ultra-thin films. In this paper we propose a new method to determine the dieleotrical coefficient and thickness of Langmuir-Blodgete film unambiguously by measuring the angle spectrum of minimum reflectivity with respect to thickness of air gap in Otto's coupling system. Experimental results for arachidic acid LB films with different number of molecular layers are presented.