Google announced Google Maps Service on June 2005, and they developed and used many technologies to solve performance balance issue that long existing in client-server GIS system, including Ajax, map data pre-processing, and browser's cookies system. OGC defined this mechanism as WMS-C. However, Ajax is a technology based on HTML and javascript, when processing mass spatial data, it could be the bottleneck of performace. Therefore, using browser plugin to improve performance on data processing is necessary. This paper discuss on WebGIS related technologies, including CGI, Java Applet, Ajax, Flash, and compare their weakness and strength, then we focus on analyzing Java Applet, Ajax, Flash. Flash provides better graphics processing abilities, smooth interactive operation, fast display performance, make it suitable to be a WebGIS platform. Flash also support multi-platform and multi-browser, and it can be deployed with the same programming code. So this study chooses Flash and it's language Action Script to develop WebGIS client, backend with Mapserver to publish map data. We also use TileCache to pre-process aerial images of National Sun Yat-sen University into image tiles. Also it is overlaid with Kaohsung district map as vector data. The actionscript in the WebGIS core library include classes like Map, View, Layer, TMSLayer, FeatureLayer. It is fully implemented as a Flash based WebGIS system. Lastly, we compare the Flash-based MapServer system with MapServer only regarding to there performance and interactivity. On loading vector data, flash performance is nearly equal to Mapserver. When vector panned, flash performs better because flash uses pre-loading image tiles, but Mapserver renders repeatly after every move. On interactivity, flash can directly zoom the map with mouse wheel, and it can shorten user¡¦s operating time. And flash can also dispaly animations like fade in, out, etc. After removing one single layer, the map still preserve the other layers, so users can handle the map information more precisely, and can reduce the loading of servers.