Between the metal melting point and the eutectic temperature of the metal-oxygen, direct bonded copper (DBC) depends on the eutectic composition to join the copper and ceramic. The key of this technology is to import the oxygen to the bonding surface. In this paper, we do some research on the preoxidation law of the copper layer in direct bonding technology, so we can optimize the rang of preoxidation temperature and time. After we investigate the phase diagram of copper-oxygen, by the "conservation of mass" and "level principle", we find there are some relationship between the preoxidation condition and the bonding process, and we offer a experiential formula to help control the bonding process. Finally, we also analyze the interface reaction production between the Cu layer and alumina ceramic, and discuss some possible reasons. (C) 2003 Elsevier Science B.V. All rights reserved.