The　effects　of　heat　treatment　on　microstructure　and　mechanical　properties　of　the　Mg-5Zn-xEr　alloys　(x　=　0.63,　1.25,　6.3　wt.%)　have　been　investigated　by　using　XRD,　DSC,　SEM　and　TEM.　The　investigation　suggested　that　the　kinds　of　the　secondary　phases　had　a　great　effect　on　microstructure　and　ageing　responses　of　the　alloys.　It　was　found　that　the　Mg-5Zn-0.63Er　alloys　which　contained　the　secondary　phase　of　I-phase　exclusively　after　solution　treated　at　480　degrees　C　fo　r　10　h　displayed　an　optimal　ageing　response　at　175　degrees　C　for　36　h.　However,　the　Mg-5Zn-1.25Er　alloy　including　both　W-phase　and　I-phase　after　solution　treated　520　degrees　C　for　10　h　exhibited　its　good　ageing　behaviors　at　175　degrees　C　for　10　h.　Meanwhile,　the　Mg-5Zn-6.3Er　alloy　after　solution　treated　at　520　degrees　Cfor　10　h　presented　its　preferable　ageing　response　at　175　degrees　C　for　18　h.　The　peak　hardness　was　ascribed　to　the　presence　of　the　beta＇(1)　phase　which　precipitated　during　ageing　process.