This paper reports on our development of a GIS-based traffic network analysis system, named GIS-based Transport Decision Support System, which provides a graphical analysis platform to transportation planners and researchers for transportation network analysis. The system has the functions of designing traffic networks on digital maps and doing traffic equilibrium analysis, as well as a novel, function to integrate local detailed structures of intersections into global networks. The latter is particularly useful for the analysis of large traffic network where the detailed local network structures of some intersections have to be taken into account. The system links great volumes of traffic data and geography information data accumulated for visualization traffic analysis. We added information data on the following: road structure data, zone geography information data and node geography information data. The system also enabled us to extract traffic data by road section and by specific condition.