In this paper, genetic algorithm (GA) tuned PID controller is presented to solve the problem of automatic generation control (AGC) of four area interconnected power system having multi-generators and diverse sources of power generation. The comparison of integral, proportional integral (PI), proportional integral derivative (PID), optimal and sub-optimal controllers reveals that the PID controller gives better system performance. Then, the parameters of PID controller are tuned using GA. The superiority of the GA tuned PID controller is shown using the eigen values, damping ratio, settling time for frequency and tie-line power deviations and dynamic responses for the deviations. Furthermore, the sensitivity analysis is carried out which demonstrates the robustness of the GA optimized PID parameters to wide variations in operating loading condition and time constants of turbine, speed governor and tie-line power. Finally, the controller is applied to a more realistic power system by considering the non-linear constraints like generation rate constant (GRC), governor deadband and time delay.