The　microstructure　of　an　as-cast　Mg-Zn-Er　alloy　was　investigated　through　scanning　electron　microscopy　(SEM)　and　transmission　electron　microscopy　(TEM)　equipped　with　energy　dispersive　spectroscopy　(EDS).　The　results　indicate　that　two　different　second　phases,　one　with　eutectoid-lamellar　morphology　and　the　other　with　granular　shape,　distribute　in　the　alpha-Mg　matrix.　The　coexistence　of　the　face-centered　icosahedral　quasicrystalline　phase　(I-phase)　and　W-phase　with　the　face-centered　cubic　structure　is　found　in　the　as-cast　alloy.　The　coexistence　of　I-phase　and　W-phase　in　the　Mg-Zn-Er　alloy　is　because　the　W-phase　is　the　primary　phase　and　the　I-phase　forms　by　peritectic　reaction　during　solidification.