This paper proposes an application of a new hybrid approach combining Genetic Algorithm (GA) and Particle Swarm Optimization (PSO) to optimal Load Frequency Control (LFC) design in interconnected power system. The proposed hybrid GA-PSO technique was applied to obtain the Proportional-Integral-Derivative (PID) controller parameters. The random nature of the GA operators makes the algorithm sensitive to initial population. However the GA algorithm may not converge if the initial population is not well selected. On other hand, PSO was shown to converge rapidly during the initial stages of a global search, but around global optimum, the search process will become very slow. The main idea of this paper is to propose a new algorithm which combines GA and PSO to solve the frequency regulation problem in interconnected power system. Time domain performance of the LFC controller, such as the maximum overshoot and settling time are being optimized based on the performance criteria like the Integral Square Error (ISE), Integral Absolute Error (IAE), Integral Time multiply Absolute Error (ITAE) and Integral Time multiply Square Error (ITSE), or their combination. To verify the effectiveness of the proposed algorithm, initially a two-area power system is used then the proposed approach was extended to the three-area interconnected power system. The obtained simulation results are compared to those yielded by the other classical and heuristic optimization techniques surfaced in the recent state-of-the-art literature. The comparative study demonstrates the validity and the potential of the proposed approach and shows its robustness to solve the optimal LFC problem.