Lead magnesium niobate titanate is an important ferroelectric material. In this study, the terahertz (THz) transmission properties of a 0.73Pb(Mg1/3Nb2/3)O3-0.27PbTiO3 single crystal were investigated using a time-domain spectroscopy method. Complex refractive index and dielectric dispersion functions were determined from the amplitude and phase information derived from time-domain responses. Based on calculations, it was concluded that the room-temperature dielectric constant of the single crystal equal to similar to 30 at 1THz. This result could be a useful reference for development of ferroelectric-material-based THz components and devices.