The　precipitation　of　the　Mg-8Gd-2Er　(wt.%)　alloy　at　200　degrees　C　was　investigated　by　transmission　electron　microscopy　(TEM)　and　X-ray　diffraction　(XRD).　The　precipitation　sequence　was　described　as　supersaturated　solid　solution　(S.S.S.S.,　cph)-＞beta　＇＇(D0(19))-＞beta＇(bco)-＞beta-phase(fcc),　which　was　different　from　previously　reported　four　stages　sequence:　(S.S.S.S.,　cph)-＞beta　＇＇(D0(19))-＞beta＇(bco)-＞beta(1)(fcc)-＞beta-phase(fcc).　The　morphologies　and　crystal　structures　of　both　beta　＇＇　and　beta＇　precipitates　in　the　alloy　were　studied　in　the　present　investigation.　The　orientation　relationship　between　the　precipitate　phases　and　the　alpha-matrix　was　observed.　The　ultimate　tensile　strength　(UTS)　and　tensile　yield　strength　(TYS)　of　the　as-cast　alloy　were　189　and　110　MPa,　respectively,　and　the　UTS　and　the　YTS　were　improved　greatly　after　ageing　treatment.　The　values　of　UTS　and　YTS　after　peak　ageing　process　were　308　and　246　MPa,　respectively.　The　improved　mechanical　properties　were　mainly　ascribed　to　the　formation　of　beta＇　phase.　The　age　hardening　response　decreased　with　increasing　ageing　time　because　the　beta＇　phase　got　coarse　as　ageing　time　increased.