Abstract: The Automatic Generation Control (AGC) of interconnected power systems has been considered as one of the most challenging problems for about the last four decades. Following the pioneering research of Elgerd and Fosha in 1970, a heap of research are appeared from time to time on optimal AGC of power systems considering various structural as well as technical aspects of power systems. This study is an attempt to propose a design of optimal AGC gains of a multi-area power system. The design of optimal AGC gains is based on Genetic Algorithm (GA) and Simulated Annealing (SA). It is a synergetic combination of GA and SA through a penalty function. The penalty function is derived based on the transient response specifications of dynamic response of the interconnected power system. A multi-area interconnected power system model is considered for the investigations. The power system model consists of three identical control areas consisting of reheat thermal plants. The investigations have been carried out with the designed Automatic Generation Controllers (AGC) based on GA-SA considering 2% load perturbation in one of the areas. The system dynamic responses obtained for various system states are compared with those achieved using optimal controllers derived using Linear Quadratic Regulator (LQR) theory. The simulation results show that the proposed GA-SA based optimal load frequency controllers are superior in all respects in comparison to the optimal AGC based on LQR theory.