Bonding phenomena with clean and smooth surfaces at room temperature (293 K) in ultrahigh vacuum has been studied. The metal surfaces, finished to required smoothness in nm level, were bombarded by Ar ions to remove surface contaminants. Then, two cleaned surfaces were put together. Interaction of silver and copper was examined by SEM, AES, and TEM. In order to observe the fundamental phenomena, bonding was attempted between a silver disk with a flat surface and a copper sphere 80 pm in diameter. A bonding area of about 40 μm2 was confirmed under slight load at 293 K. Strong bonds were achieved and the fracture took place almost completely in silver bulk. Furthermore, it was found that the bonding area obtained by the process was much larger than the one derived from conventional plastic theory.