In　order　to　save　the　fuel　consumption　and　reduce　the　pollution,　aluminum　alloy　is　one　of　the　most　acceptable　materials　for　weight　reduction　in　vehicle　body　assembly.　However,　how　to　join　aluminum　alloys　and　steels　is　an　unsolved　problem.　Friction　stir　welding　is　tried　to　join　aluminum　alloy　and　zinc　coated　steel.　The　steel-aluminum　dissimilar　materials　lap　joints　are　manufactured　by　friction　stir　welding　in　two　ways.　One　is　that　the　pin　is　pressed　into　the　steel　material　to　cause　a　mechanical　and　metallurgical　bonding　between　the　dissimilar　materials.　Another　is　that　only　a　metallurgical　bonding　between　the　dissimilar　materials　is　formed　by　the　heat　flow　from　the　friction　of　the　tool　and　the　aluminum　material.　The　lap　joints　are　shear　tested　and　the　interface　between　steel　and　aluminum　materials　are　observed.　It　is　shown　that　the　zinc　coating　on　the　steel　plate　has　an　obvious　effect　on　the　joint　forming　between　steel　and　aluminum　and　there　is　an　obvious　difference　in　shear　strength　and　failure　mode　between　metallurgical　and　mechanical-metallurgical　bonding　joints.　In　mechanical-,　metallurgical　bonding　joint,　the　interface　layer　between　steel　and　aluminum　materials　is　thick　and　composed　of　aluminum,　zinc　phase　while　in　metallurgical　bonding　joint,　the　thickness　of　interface　layer　is　about　2-3　μm　and　it　is　mainly　composed　of　the　intermetallic　compound　of　Fe-Al.　And　the　factors　influencing　joint　strength　and　failure　mode　of　steel-aluminum　friction　stir　welding　joint　are　also　investigated.　©,　2015,　Journal　of　Mechanical　Engineering.　All　right　reserved.