Distributed experts united system is an advanced topic in knowledge engineering. In this paper, the design thought, objective, architecture, principle of UNION which is a distributed experts united system on microcomputer net are described. The concept of distributed experts united system was first proposed by professor Lu Ruqian in 1985. In the next year, the experimental system UNION began to be developed by Institute of Mathematics, Academia Sinica. The environment is UNOS system with C and PASCAL on MC68000 microcomputer.Three microcmputers are connected by an Ethernet.So far,the supporting system Support 1 and communication system have been finished and other units are being developed.