In multi-objective optimization， Pareto front can offer valuable tradeoff information between objectives， which is the basis for decision-making. Normal Boundary Intersection（NBI） is an effective method for evenly generating Pareto optimal solutions. In this paper， NBI method is introduced into generation dispatch， and another valuable index， i.e. degree of substitute index. Its expression is derived based on the NBI method. This index can intuitively reflect the substitute relation between the generation cost and loading margin， providing the decision-maker with important reference to determine the optimization scheme. Simulation results of a numerical example illustrate the validity of the method.