Power system stabilizing control plays an important role to remain in synchronism during major disturbances resulting from sudden or sustained load changes, either the loss of generating or transmission facilities. In this paper, two methodologies to allocate TCSC (Thyristor-controlled Series Capacitor) controllers have been applied to stabilize disturbances in power systems. One takes steady-stability of power system into consideration. The other considers transient stability. Both methodologies figure out location points and the number of TCSC controllers satisfying the stability index. After allocating TCSC controllers, this research applies robust controller design for them. As the methodology of robust controller design, output feedback H_∞ control has been utilized. Nowadays, LMI (Linear Matrix Inequality) approach is very effective, since it solves the Lyapunov inequality without considering the weighting coefficients used in other control theories. This paper has designed the robust TCSC controllers for multi-machine systems based on the LMI approach. In the proposed approach, input information of TCSC controllers is only angular velocity deviations of generators as observable value. The proposed LMI-based approach is proved to be applicable for the design of robust TCSC controllers to enhance robustness and fast response by simulations on a test system.