Economic Load Dispatch (ELD) is one of the optimization problems in power systems. Economic Load Dispatch determines the electrical power to be generated by the committed generating units in a power system so that the total generation cost of the system is minimized, while satisfying the load demand simultaneously. In this paper Artificial Immune System (AIS) based optimization approach has been applied to solve the constrained Economic Load Dispatch with valve-point effect. The developed AIS based optimization technique used total cost of generation as the objective function and represented it as the affinity measure. For illustrative purposes, the proposed AIS based technique has been applied to various test cases (consisting of 3, 13 and 40 generating units) to validate its effectiveness. The developed AIS based optimization technique (both binary coded and decimal coded) has been thoroughly investigated by varying the population size. The results of the proposed AIS based optimization technique are compared with that of Genetic Algorithm (GA) based approach. The simulation results reveal that the developed technique is easy to implement and capable of finding feasible near global optimal solution. The results substantiate the robustness, fast convergence and efficiency of the proposed methodology.