We have proposed a simple reflection method to evaluate the optical constants and optical thicknesses of various films. The optical constants and thicknesses. of alkyl-urethane-substituted polydiacetylene (PDA-C4UC4) films on a BK-7 glass prism have been measured using the reflection method proposed in this paper. Unoriented blue-form PDAs have an optically anisotropic nature and their optical constants have been estimated in the tangential direction and in the direction normal to the film surface. Our reflection method also provides information on the polymerization state.