A computer method for comprehensive comparison between EHVAC and EHVDC is presented in this paper after analyzing the fundamental characteristics and studying the design methods of these two types of transmi-sson schemes. Some formulas are developed and an appropriate mathematical model is established. A generally applicable computer program is exploited which is acceptable to IBM-pc and its compatible micro-computers. The results of numerical practical project of the program are given to prove the correctness of the model and the program described in this paper,and it also shows that the method proposed is an efficient one with the practical values in power system planning.