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Web Map Service (WMS) provides spatial information to Internet users by map images. Commonly, spatial data are stored as vector data. It is a long job to create map images from vector data. To reduce the time cost, we employ Linux cluster. When a map is requested, coordinates scope that is a rectangle defined with xmin, ymin, xmax, ymax must be specified. We design a load balance algorithm to divide the request rectangles into some sub-rectangles. Each sub-rectangle is sent to a WMS node to produce some sub-maps. The resulting map will be reconstructed by these sub-maps. All these sub-maps are produced in parallel, so less time is spent in producing an entire map. How to split request rectangles is the key problem. First, we present a method to calculate the 2D load weights distribution in the map scope. Second, nodes load abilities are evaluated. Then, the authors present a method for splitting the entire rectangles. This paper also discusses the performance of the algorithm.