Gravity　has　a　significant　influence　on　weld　performance　during　variable　polarity　plasma　arc　(VPPA)　welding　of　aluminum　alloys.　Micro-structures,　residual　stress,　hardness　and　tensile　strength　of　welded　joints　using　vertical-up　and　horizontal　welding　have　been　investigated　and　discussed.　Micro-structures　and　residual　stress　in　the　welded　joint　for　vertical-up　welding　were　symmetrical.　This　is　in　contrast　to　the　horizontal　welding,　which　tends　to　have　asymmetric　and　inhomogeneous　micro-structures　that　lead　to　poor　mechanical　properties.　In　the　upper-side　of　welded　joints　for　horizontal　welding,　larger　grain　sizes　brought　about　lower　hardness　and　lower　tensile　strength.　For　the　horizontal　welded　joint,　the　grain　size　near　the　fusion　line　(FL)　on　the　upper-side　was　about　113.9　+/-　28.8　mu　m　and　the　tensile　strength　was　89.2%　of　the　base　metal　(BM),　while　for　the　veritcal-up　welded　joint,　the　grain　size　was　about　66.9　+/-　16.2　mu　m　and　the　tensile　strength　was　98.0%　of　the　BM.　On　the　horizontal　welded　joint,　the　fracture　section,　which　had　many　deep　dimples　and　tear　ridges,　appeared　only　on　the　upper-side　of　welded　joint.　The　difference　of　mechanical　properties　between　bilateral　sides　was　caused　by　gravity　driving　the　melting　metal　flow.　Finally,　an　optimized　mechanism　of　symmetrical　metal　flow　based　on　the　upper　wire　feed　position　was　proposed.　The　revised　welded　joint　was　improved　significantly　and　the　residual　stress　on　both　sides　was　symmetric.