Specimens of aluminum (Al), austenitic stainless steel (SUS304L), pure iron (Fe) and oxygen-free high conductivity copper (Cu) were treated by argon ion bombardment, and then were bonded by diffusion bonding method. The effects of argon ion bombardment on diffusion-bonded joints of four kinds of metallic materials were compared from the tensile strength at real bonded area and the fractographs. The results showed that bonding temperature was lowered by argon ion bombardment treatment for four kinds of materials. The effect of argon ion bombardment on diffusion-bonded joint depended strongly on the chemistry of the surface to be bonded, and increased in Al, SUS304L, Fe, and Cu in turn.