Room temperature bonding between two flat wafers using thin metal films was studied. Thin nanocrystalline metal films (0.2-20 nm thick) were fabricated on two flat wafers' surfaces using sputter deposition. Bonding of the two metal films on the wafers was accomplished immediately after film deposition in vacuum. The wafers were bonded at room temperature over the entire bonded area using metal films of 16 kinds including W films. The two films' bonded structure was related closely to the self-diffusion coefficients of the metal films. The very high atomic diffusion coefficient at the grain boundaries and film surfaces is likely to have enabled bonding at room temperature. The wafers were bonded even with films of 0.2 nm thickness on each side. The potential of the bonding for applications to device fabrication is examined. Moreover, bonding in air using noble metal films is examined according to experimental results.