This paper presents a new approach to solving the Economic Dispatch (ED) Problem for a large number of generators using a decomposition / aggregation method. A program has been developed to demonstrate the algorithm using the MATLAB programming language. A 5-bus test system and the IEEE 26-bus test system are used as demonstration systems. Each test system is decomposed into small areas and each area has been solved for Economic Dispatch (locally) using an Evolutionary Programming (EP) technique. It was ensured that each area contains at least one generating unit and one supplied load. The EP will minimise the objective funtion for each area, minimising the local operating cost including the effects of real power losses in each area. The optimisation problem for each area can be regarded as a sub-problem of the decomposition scheme. Subsequently, the solutions from the areas are combined (aggregated) to solve the overall system problem. The results obtained using the decomposition / aggregation method are compared with the results found when the ED Problem was solved using a centralised EP approach and the base-case results found from solving a (non-optimal) load flow. It was found that applying the aggregation method is a prospective approach for solving economic dispatch problems with a large numbers of generators in a power system.