By using backward-wave oscillator (BWO),the transmission spectra of various resistivity GaAs were tested.Then the transmission spectra were examined and analyzed.The refractive index,absorption coefficient,and dielectric functions of various resistivity GaAs and terahertz dielectric properties of GaAs were measured and compared in the frequency range from 0.23 THz to 0.375 THz.The experimental results indicate that all the refractive index,absorption coefficient,and dielectric functions of the various resistivity GaAs increase with the increase in frequency.The absorption coefficient of high resistivity GaAs is very small,and its least absorption coefficient equals 3.87×10^-4 cm^-1.The authors’ results demonstrate the applicability of the backward-wave oscillator THz transmission spectroscopy for GaAs characteristic analysis by calculating the reflectance spectra.This work establishes the basic spectra data for the various resistivity GaAs and is very significative to the design of high efficiency terahertz wave antenna.