A power system stabilizer design method for multi-machine power systems is proposed in this paper. In this design method, the design problem is translated into an equivalent problem of decentralized controller design for Multi-Input Multi-Output (MIMO) control systems. Subject to a condition based on the structured singular values, each stabilizer can be designed independently. The robust stability condition for power systems with stabilizers on can be easily stated as to achieve a sufficient interaction margin defined in this paper, and a sufficient gain and phase margin defined in the classical feedback theory during each independent design. Within this general framework, the conventional stabilizer design methodology based on the concept of synchronous and damping torques is used to decide the design details of each stabilizer. The suggested design method is applied to a model of a practical 10 machine power system.