Transmission and reflection measurements on various polymer films were performed in the THz frequency region, and the dielectric function was calculated by the Debye model to interpret the data. The ε 0 and ε ∞ obtained in our analysis show good agreement with the ε 0 from the electric measurements in MHz–GHz frequency region, and the ε ∞ calculated from the refractive index in the visible region. The abnormal dispersion was confirmed to appear in the THz frequency region, and the dispersion was not observed in non-polar polymers. This evidence shows the dispersion is due to the orientation polarization of polymers.