Digital Earth is a kind of GIS application which use lots of computer techniques, including ¡§Computer Graphic¡¨, ¡§High quality Satellite Image¡¨, ¡§Multi-resolution¡¨, ¡¨ Metadata¡¨, ¡¨Level of detail¡¨, etc. In the past, geographers always tend to use triangle or grid structure to build 3D GIS. Now 3D GIS tend to use VRML to display virtual terrain data. Nevertheless, this display method is short of three-dimension analysis. It just uses 2.5 dimension to simulate 3 dimension space structure. This thesis is base on the theory of GeoVRML Quad-tree structure and use JAVA to implement ROAM algorithm. And this method can improve Quad-tree structure morphing error and raise virtual terrain display speed. This thesis use ETOPO5 5-minute DTM data to build Earth model (provide by NGDC) and Taiwan 40M DTM data, 200M, 100M, 50M, 25M satellite image to build Taiwan 3D GIS model. Meanwhile, this thesis addresses an unique three level structure to display 3D GIS model. One is Satellite Image layer, one is virtual terrain layer, and last one is 3D feature object.