This study was investigated the influence of preheat temperature and welding speed by a view point of microstructure of joining zone used by friction stir welding (FSW) between ductile cast irons and stainless steels. FSW conditions were carried out by change of tool rotation by 200, 400 and 600rpm, preheat temperature by non-preheat, 573 and 773K, and welding speed by 1～10mm/s. As a result, microstructure of FCD450 side became a pearlite structure changed by preheat condition. Also, stir zone (SZ) of FSW were crystallized the chill structure because of cast iron melted partially why SZ temperature exceeded the eutectic temperature. However, higher welding speed could inhibited crystallization and found the possibility of bonding between ductile cast irons and stainless steels.