The change of the dielectric constants and thicknesses has been measured in-situ on alkyl-urethane-substituted polydiacetylene (PDA) films grown on a silver (Ag) film during a photopolymerization and photochromic change by using the attenuated total reflection geometry. Photopolymerization and photochromic change in the vacuum-deposited PDA films from the monomer to the blue, the red, and the bleached forms, have been readily achieved by ultraviolet light irradiation with a D-2 lamp. Surprisingly, the values of the dielectric constant of the thin PDA films deposited on the Ag film were found to be comparable with those of oriented films. The dielectric constants of PDA films grown on a BK7 glass have been measured by using a normal-reflection method in the wavelength region from 400 to 1400 nm at room temperature.