Welding between ADC12 and SS400 was successfully performed by friction stir diffusion process. The experiments were conducted by a machining center with a cemented carbide tool and welding conditions, such as a rotating speed, welding speed and position of the tool, were optimized. By plunging the rotating tool into the interface with offset, ADC12 and SS400 were welded above 2000 rpm of the rotating speed at the welding speed of 50mm/min. Tensile strength was evaluated for the welded specimens. The specimens fabricated above 5000rpm in the rotating speed were fractured at the weld zone and showed relatively low tensile strength. The specimens, on the other hand, made between 2000 and 4000 rpm were fractured in the portion of the base aluminum alloy. Achievement of more than 100% in joint efficiency was indicated in the present friction stirring process.