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Various operating conditions, nonlinearity effects etc. cause several system uncertainties. The conventional lead/lag power system stabilizers (PSSs) which are designed without considering such uncertainties may deteriorate a system robust stability. As a result, PSSs may fail to damp electromechanical oscillations. To tackle this problem, this paper focuses on a new decentralized control design of robust PSSs considering system uncertainties. Scalar uncertainty models are included in a system representation. Accordingly, the stability margin of the controlled system can be determined in terms of the guaranteed multi-input multi-output (MIMO) gain margin (GM) and phase margin (PM). To achieve the guaranteed MIMO GM and PM, the tabu search is employed to search for optimum parameters of PSSs. The objective function is formulated based on a minimization of infinite norms of a closed-loop control system and a complementary sensitivity function. Consequently, nonlinear simulation studies of the New England power system confirm the robustness of the designed PSSs against system uncertainties.