Abstract. Multi-user virtual environment (MUVE) systems enable virtual participation in many applications. A MUVE usually is a complex system requiring technologies from 3D graphics and network communication. However, most current systems are designed to realize specific application contents and usually lack system extensibility. In this paper, we propose an extensible architecture for a client-server based MUVE system called IMNET. This XMLbased MUVE system allows function modules to be flexibly plugged into the system such that network or user interface experiments can be easily incorporated. We will use two examples to illustrate how to flexibly change the system configurations on the server and client sides to enhance system functions or to perform experiments. We believe that such an experimental test-bed will enable a wider range of researches to be carried out in a more efficient way.