The　effects　of　homogenization　and　isothermal　aging　treatment　on　the　mechanical　properties　of　Mg-12Gd-2Er-1Zn-0.6Zr　(wt%)　alloy　were　investigated.　The　precipitated　long-period　stacking　order　(LPSO)　structure　and　the　aging　precipitation　sequence　of　the　conditioned　alloys　were　observed　and　analyzed,　respectively.　The　results　indicate　that　the　14H-LPSO　structure　occurs　after　the　homogenization　treatment　and　the　beta＇　phase　forms　after　the　isothermal　aging　process.　These　two　independent　processes　could　be　controlled　by　the　precipitation　temperature　range.　The　significant　increase　in　the　elongation　of　the　as-cast　alloy　after　homogenization　treatment　is　attributed　to　the　disappearance　of　the　coarse　primary　Mg-5(Gd,　Er,　Zn)　phase　and　the　presence　of　the　14H-LPSO　structure.　The　precipitation　sequence　of　the　investigated　alloy　is　alpha-Mg(SSS)/beta＇＇(D0(19))/beta＇(cbco)/beta.　Furthermore,　the　yield　tensile　strength　(YTS)　and　ultimate　tensile　strength　(UTS)　values　of　the　isothermal　aging　alloy　have　a　great　improvement,　which　could　be　attributed　to　the　high　density　of　the　precipitated　beta＇　phase.　With　the　aging　time　prolonging,　the　beta＇　phase　is　precipitated　in　the　alloy.　Both　the　14H-LPSO　structure　and　the　beta＇　phase　coexist　in　the　alloy　after　aging　for　84　h　at　498　K.　The　layer　structures　of　14H-LPSO　are　obviously　observed　along　[110](Mg)　direction,　which　is　shown　in　the　high-resolution　transmission　electron　microscopy　(HRTEM)　image.　The　14H-type　structure　displays　a　stacking　order　with　a　period　of　c　=　3.67　nm,　which　does　not　change　during　the　isothermal　aging　process.