The partitioning level of a catchment in storm water runoff modelling becomes an issue if the calculated results from the different number of subcatchments show the same performance. This study aims to identify the proper processing level of spatial resolution for the SWMM model application in an urban area. Using GIS overlaying technique, the division of subcatchments as a hydrologic similarity unit (HSU) is achieved with a comprehensive consideration of surface slope conditions, flow directions of storm sewers, and current land cover situation. Three surface-sewer altematives are made on the basis of three different levels of surface divisions as well as the number of sewer connections and used as runoff simulation fields for the application of SWMM. As the result, it is found that the effect of a spatial resolution on the surface runoff results is not significant. On the other hand, the accumulated pollution load from a unit subcatchment, which is built by aggregation of several unit subcatchments consisting of various land cover conditions, is reduced through the deterioration of surface spatial resolution. Although overall runoff pattern and accumulated runoff are slightly affected by spatial resolution, the simulated runoff from sewer outlet shows slight difference at the peak appearance time.