We report the study of far-infrared dielectric properties of a polar crystal using the THz time domain spectroscopy. We measured the power and phase transmission spectra by coherent THz radiation. The anisotropy of dielectric properties has been investigated simply by rotating the polarization plane of an incident beam. The frequency dependence of complex dielectric constant has been determined without the Kramers-Kronig analysis. The complex dispersion relation of phonon polariton frequency and damping has been also determined. These far-infrared properties have been consistently reproduced by the damped harmonic oscillator model related to the lowest frequency polar mode.