Optical bistable devices have attracted much attention for application to optical-optical switch and so forth. We have prepared an optical bistable device of quasi-waveguide constituted comprise of a triangular prism and a poly (3-hexylthiophene)/poly (methyl methacrylate) composite thin film. The thin film was prepared by the spin coating method and drop casting method. With both the thin films, optical bistability was clearly distinguished. The optical bistable characteristics of the quasi-waveguide prepared by the spin coating method can be observed with lower laser power intensity than that by the drop casting method.