Microstructure　evolution　of　the　cast　Mg-9Gd-2Er-0.4Zr　alloy　during　solid　solution　treatment　at　temperature　of　460-520　degrees　C　for　3-12　h　was　investigated　by　using　optical　microscope　(OM),　scanning　electron　microscope　(SEM)　and　transmission　electron　microscope　(TEM).　The　results　indicated　that　the　grain　size　and　the　shape　of　second　phase　were　obviously　changed　with　time　and/or　temperature　going　on.　At　460　degrees　C　for　3　h,　the　morphology　of　the　Mg-5(GdEr)　phase　was　changed　into　fragmentized　island　morphology　and　the　volume　faction　of　the　phase　decreased.　After　solution　solid　treatment　at　460　degrees　C　for　6　h,　the　Mg-5(GdEr)　phase　was　already　completely　dissolved,　but　some　cuboid-shaped　RE-rich　phase　precipitated.　As　the　temperature　increased,　the　morphology　of　the　Mg-5(GdEr)　phase　was　transformed　into　the　same　morphology　as　that　at　460　degrees　C　for　6　h.　It　was　suggested　that　the　microstructure　evolution　of　the　alloy　during　the　solid　solution　treatment　was　concluded　as　follows:　Mg-5(GdEr)　eutectic　phase　-＞　Gd/Er　atom　diffusing　into　matrix　-＞　spheroidic　Mg-5(GdEr)　phase　-＞　cuboid-shaped　RE-rich　phase　-＞　grain　boundary　immigration.