Transmitted terahertz time-domain spectroscopy (THz-TDS) using dipole-antenna on LT-GaAs was applied to determine the THz spectral characteristics of single-crystal ZnO, pure-and Mg-doped ZnO nanopowders (NPs) at room temperature. The power absorption and refractive index were measured in the frequency range from 0.2 to 2 THz. Mg doping increases the absorption coefficient, while decreases the refractive index of the compound ZnO NPs. The THz dielectric properties of pure-and Mg-doped ZnO NPs exhibit similar behaviors with that of the ZnO single crystal, which are found to be associated with the transverse optical E1 (TO) phonon mode.