Two kinds of semiconductor substrates, n-Si and p-Si, are investigated by the terahertz (THz) time-domain spectroscopy. The reference signals of free space and the sample signals are measured, and a series of optical parameters of these two samples in the range of 0.5～2.0 THz are calculated. The refractive index and real part of complex relative permittivity of n-Si are independent of frequencies, as well as p-Si. The extinction coefficient and imaginary part of complex relative permittivity of p-Si decrease with the increasing frequency, and this phenomenon is much more obvious than that of n-Si. The complex conductivity and dielectric loss also have been calculated. The obtained results indicate that n-Si and p-Si have good dielectric properties and can be used to work in the THz range.