Abstract: This paper presents the design algorithm of decentralized output feedback control of large scale systems and its application to the stability control of multi-machine power systems.An equation is proposed to express the information constraint,so the control problem is transformed into a restricted static optimization problem.The optimal condition and gradiant formula of linear system are conveniently obtained,and the problem is solved by the BFGS method.A control scheme of power system is put forward,in which the deviations of power,voltage,speed etc.are selected as the local feedback signals and simple decentralized controllers are used.For a typical system the control scheme is studied and compared with classical propotional,nonlinear optimal and linear optimal control schemes.The results show that the proposed decentralized output feedback control scheme is simple,effective and adaptable.In nonlinear system,appropriate decentralized out put feedback control is not only simpler but also more effective than linear optimal state feedback control.