The terahertz (THz) refractive indices of a liquid crystal 4'-n-pentyl-4-cycazobiphenyl (5CB) in both of the nematic and isotropic phases were deters mined by terahertz (THz) time-domain spectroscopy for the first time, to our knowledge. We find that the extinction coefficient of 5CB is negligibly small in this frequency range. The birefringence of 5CB, on the other hand, is as large as 0.2. This is comparable to that of 5CB in. the visible and near M. The experimental results indicate that liquid crystals are potentially useful for device applications in the THz frequency range.