The　mechanism　of　the　microstructure　refinement　of　metals　and　alloys　(5052　and　6061)　by　arc　oscillating　was　studied　and　discussed　via　the　grain　size　of　regular　welds　and　those　welded　at　different　oscillation　frequencies.　In　5052　alloy,　fine　columnar　crystals　appeared　near　the　＇internal　fusion　line＇　during　the　oscillating　arc　welding　and　the　grains　have　been　significantly　refined　by　oscillating　arc　with　low　and　high　frequency.　However,　the　grain　refinement　of　6061　Al　alloy　was　limited　during　arc　oscillating　welding　due　to　the　different　crystal　growth　pattern　at　the　fusion　line　and　material　properties.　It　is　not　caused　by　changing　of　the　grain　refinement　mechanism　but　by　the　difference　in　the　material　properties.