This paper presents a real-time economic dispatch method by calculating the penalty factors from a base case data-base.The base case data-base is established from some operating points of the seasonal average of daily demand curve.Real-time basis equation derivation is performed, and the solution procedure is described.An approximate solution can be obtained very quickly by the proposed method. An example system is tested to show the computation capability of the proposed method.The computation time of this method is much shorter, and the solution accuracy is good, compared to other adopted methods. This method is very suitable for real-time economic dispatch in power system operation.