Abstract: In the design of decentralized controllers for large scale systems, the system performance is usually not good enough. Sometimes the whole system may be unstable if all the interactions between the subsystems are neglected, while a problem of high demension (equals to the order of the whole system) has to be solved if the interactions are fully considered, which is not realizable in many cases. A simplified approach is proposed here. The decentralized controller of a subsystem is designed using the reduced order model for the rest of the system. In other words, the subsystem and the order reduced model are taken to compose an approximated low order system, upon which the decentralized controller for the subsystem is designed. And it is shown that the approach could greatly simplify the design of decentralized controllers for high demension systems and achieve high performance.