The　microstructure　evolution　of　the　Mg-3Zn-0.5Er　-0.5Al　(mass　fraction,　%)　alloy　under　the　different　condition　was　investigated.　The　results　showed　that　as-cast　Mg-3Zn-0.5Er-0.5Al　alloy　mainly　consisted　of　primary　large　irregular　Mg4Zn7　phase　and　needlelike　(Mg,　Zn,　Er,　Al)　quaternary　phase.　Mg4Zn7　phase　almost　dissolved　into　the　matrix　after　solid　solution　treatment　at　400　°C　for　10　h,　while　the　(Mg,　Zn,　Er,　Al)　quaternary　phase　still　existed.　The　solution　treated　alloy　was　extruded　at　250　°C.　The　ultimate　tensile　strength　of　the　as-extruded　alloy　was　approximately　268　MPa　and　the　YTS　was　approximately　163　MPa　companying　with　an　elongation　of　28%.　The　tensile　strength　of　the　as-extruded　alloy　improved　obviously,　which　was　mainly　attributed　to　the　grain　refinement.　©　(2015)　Trans　Tech　Publications,　Switzerland.