This paper presents a completely decentralized scheme for the robust control of multi-machine power systems with a prescribed degree of stability. By applying the optimal control technique to the nominal dynamics of each decomposed machine, the calculated optimal gains are multiplied by appropriate factors to account for the intrinsic uncertainties and nonlinear interactions among the machines. This subsystem-based method can give the overall system an arbitrarily chosen degree of stability, where no state information is transferred. An example of a three-machine power system is given below.