Electroabsorption spectra of Parylene-C films are found to exhibit several peaks whose wavelengths depend on the thickness of the film. The spectral feature is well correlated to the first derivative of the transmission spectrum of the Parylene-C film sandwiched between two reflective metal layers, and the peak intensities are in proportion to the square of the magnitude of a modulation voltage. The electroabsorption spectra observed in a transparent region of the electrically insulating films are accounted for in terms of the interference effect coupled with the electro-optical Kerr effect of the Parylene-C films.