Abstract: The key to combining multimedia computation with electronic mailing system is to build a multimedia user agent. In this paper, we first extend the layer model based on X. 400 series to support multimedia message. Then we propose a method for message transformation based on presentation schema in heterogeneous environments and a multimedia information model that is concerned with temporal and spatial features of multimedia composition and synchronisation as well as interaction with the user. Finally, the overall design and implementation of a multimedia user agent TH-UA is described.