This paper presents a diffusion bonding technology at vacuum using a high-boiling liquid protection, diethylene glycol dimethyl ether (C6H19O3). After washing in alcohol, the specimens were immersed in diethylene glycol dimethyl ether. Before heating, the vacuum chamber was vacuumized to about 10-1Pa. Then chamber was heated to about 80°C, and held for 20 minute to evaporate the liquid protection in order to make the clean bonding surfaces contact. Metallographic observations of the interface by optical microscope were carried out. The experimental results show that the protections have successfully protected the bonding surfaces from re-oxidation prior to bonding. The metallographs show that the vacancies at the bonding interface bonding at 560°C for 2 hours are obvious and the materials at the interfaces of specimens bonded under the pressure of 5 MPa at 535°C for 2 hours were well joined.