A new approach based on sensitivity method for the optimal power flow is presented in the paper. This method has the following advantages: it is suitable to solve a 200-node power system on the microcomputer; the convergence is easy to reach and the computation speed is quick; it is possible to reduce 8-14% power loss and to get maximum economic benefit by using this method; in power system dispatch center it can be used as an on-line voltage and reactive power control program to keep good voltage level. The proposed method has been proved successful by IEEE 118-node test system and the several real power systems such as 49-node, 61-node, 99-node and 200-node systems.