In this work a recently reported novel meta-heuristic optimizer called grey wolf optimization(GWO) has been applied to optimize the weighty parameters in automatic generation control(AGC) of interconnected three unequal area thermal system, such as gain constant of PID controller (Kpi, KIi, Kdi). A three unequal area interconnected system is studied as a test system under different loading conditions, the nonlinearities nature of the system are considered into account to confirm the effectiveness of the grey wolf optimization in terms of suitable generation rate constraints(GRC). The result of GWO based PID controller is compared with bacteria foraging based optimization technique [5].