Substation optimization planning plays an important role in urban network planning. This paper presents a novel method for substation planning based on the weighted Voronoi diagram， which can optimize the number， capacity， location， size and power supply area of substations. First， a mathematical model is proposed to obtain the capacity combination of the new substations， in which the optimum solution and sub-optimal solutions can be obtained simultaneously by integer programming. Then， two methods are adopted to get the initial locations in terms of the existence of substations. One is the ordinary Voronoi diagram method and the other is the geometric method that takes the local topographic characteristics， the area of the planning district and load distribution into consideration. The uneven load distribution， the different rated capacities and the load rates of each substation can seriously affect the power supply area. In view of these effects， a substation optimization method based on the weighted Voronoi diagram is proposed to automatically decide the power supply area of each substation and optimize the locations of the new substations. Actual examples show that the method proposed is effective.