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The stabilization of power system oscillations between interconnected areas is very essential for secure system operation. Although PSS and FACTS devices have been applied to multi-machine power systems for enhancing system stability, using only conventional PSS may not provide sufficient damping, particularly for inter-area oscillations. In these circumstances, FACTS controllers are expected to be effective solutions. Furthermore, in recent years, with the deregulation of the electricity market, the traditional concepts and practices of power systems that new construction of equipment should be continued to maintain supply reliability have changed. Better utilization of existing power systems to increase capacities by allocating FACTS devices becomes imperative. However, FACTS devices, such as TCSC, have been used conventionally to regulate local mode oscillation. Coordinated control of PSS and FACTS devices is proposed to improve the inter-area mode oscillation at minimum cost. The approach presented enables us to allocate a set of TCSCs to improve the stability of interconnected power systems. The proposed approach determines the location of controllers by making use of eigenvalue analysis, active power sensitivity and linear quadratic regulator. Moreover, this approach adopts the H infinity norm as a stability index for optimally allocating controllers.