In this paper, a novel approach based on stochastic particle swarm optimization (SPSO), with dominant eigenvalue shift for designing robust decentralized load frequency control system for interconnected power system is presented. A proportional - integral-derivative (PID) controller is considered to exemplify the optimum parameter search. To demonstrate the robustness of the obtained controller, a two-area nonreheat thermal system, equipped with such optimized tuned controller, is tested under different operating conditions and parameter changes. The optimal PID parameters search is formulated as an optimization problem with a standard eigenvalue objective function. Its effectiveness is shown through comparison with the well-known conventional integral controller. The eigenvalue-based performance index is considered. The simulation results demonstrate the enhancement in the dynamic response of the two area power system.