The semi-empirical dispersion formula was applied to analyze the mode dispersion and Cerenkov radiation attenuation characteristics of coplanar striplines on LT-GaAs substrate in the frequency domain over a terahertz range. Also,FFT was employed to numerically model the time domain propagation characteristics of ultrashort electrical pulse along the coplanar striplines on LT-GaAs substrate. The results reveal that distortion will appear in the waveform of the ultrashort electrical pulse because of the mode dispersion and Cerenkov radiation attenuation mentioned above at frequency range over THz. The shorter the width of the original pulse is,the more serious the distortion becomes. The reduction in the thickness of the LT-GaAs substrate can weaken the mode dispersion and Cerenkov radiative attenuation,and as a result,the propagation characteristics of the ultrashort electrical pulse along the coplanar striplines on LT-GaAs substrate is improved.