THz(teraherz)properties of high-Tc superconductor substrates(100)MgO and (100)LaAlO3 are measured using time-domain spectroscopy at frequency range between 0.2 and 2 THz.The complex refractive index and dielectric function of the MgO substrate is independent of frequencies;those of LaAlO3 increase slightly with increasing frequency.We determine=3.46+i0.001 and=12.27+i0.06 for the MgO substrate and =4.78+i0.02 and =22.5+i0.2 for the LaAlO3 substrate at 1THz.The dielectric loss tanδ of both LaAlO3 and MgO substrate is less than 0.01The tanδ of LaAlO3 in nearly five times larger than MgO,but both of them are less than 0.01.