Automated industries require high quality of power supply for better performance. Therefore, power generating units play crucial role. In this work, multi-area power system incorporates six area thermal power systems with different steam configuration is investigated with Automatic Generation Control (AGC). The investigated system included non-reheat thermal power system in area 1 and 2, while area 3 and 4 are single reheat thermal power system and area 5 and 6 are double reheat thermal power system. All areas are interconnected through tie-line. A Proportional-Integral-Derivative (PID) controller is proposed as a secondary controller to provide the necessary control signal during sudden load demand. To achieve optimal PID controller performance, the controller gain values are optimized by considering nature inspired meta-heuristic algorithm, namely the Ant Colony Optimization (ACO) technique using different objective functions. Time domain specification analysis is considered to evaluate the performance of investigated power system, where the power system is designed and modeled under Matlab/ Simulink environment. Finally, simulation results established that Integra Time Absolute Error (ITAE) objective function based PID controller provided superior performance compared to other objective function based PID controller.