Aluminum　alloy　and　steel　thin　sheets　have　been　mostly　used　in　the　automotive　industry　to　get　a　lightweight　car　body.　Nowadays　several　studies　are　focused　on　the　joining　of　aluminum　alloy　to　steel　by　new　welding　methods　especially　by　laser　welding.　In　this　work　dual-beam　fiber　laser　keyhole　welding　was　introduced　to　joining　of　1.5　mm-thick　aluminum　alloys　to　1.8　mm-thick　304　stainless　steels　in　an　overlap　joint　configure.　The　influences　of　different　laser　focusing　positions　on　the　weld　appearance,　interface　microstructures　and　tensile　mechanical　resistance　of　the　welded　joints　were　studied.　As　a　result,　the　good　weld　appearance　of　the　aluminum　alloy　to　stainless　steel　joints　were　obtained　by　dual-beam　fiber　laser　keyhole　welding　process　without　any　filler　materials.　The　thickness　of　the　intermetallic　compound　layer　of　the　joint　interface　is　comparatively　thin　when　the　laser　beam　with　low　energy　is　focusing　on　the　front.　The　nano-hardness　testing　results　show　that　the　average　hardness　of　intermetal-lic　compound　layer　is　9.61　GPa,　which　is　significantly　higher　than　that　of　the　parent　stainless　steel　of　4.12　GPa　and　aluminum　alloy　of　1.09　GPa.　The　fracture　of　the　welded　joints　occurs　on　the　aluminum　alloy/stainless　steel　interface　layer.　The　highest　mechanical　resistance　of　131　N/mm　can　be　obtained　by　the　low　energy　laser　beam　focused　on　the　front.