The　microstructure　and　mechanical　properties　of　the　cast　and　extruded　Mg-12Zn-1.5Er　alloys　were　investigated.　The　I-phase　observed　in　the　cast　Mg-12Zn-1.5Er　alloy　was　broken　during　hot　extrusion.　The　microstructure　of　the　alloy　was　refined　due　to　the　dynamic　recrystallization,　and　the　equiaxed　grains　have　size　in　the　range　of　2-5　mu　m.　Moreover,　a　great　deal　of　nano-scale　particles　precipitate　in　the　recrystallized　grains.　Compared　with　the　cast　one,　the　extruded　alloy　shows　a　great　improvement　on　the　mechanical　properties　as　the　result　of　refined　microstructure,　the　dispersed　I-phase　and　the　fine　precipitates.　The　ultimate　tensile　strength　and　the　yield　tensile　strength　of　this　extruded　alloy　are　359　and　318　MPa,　respectively.