The　microstructure　and　creep　behaviors　of　cast　Mg-xZn-yEr　(x=3,6,9　wt.%,　x/y=6)　alloys　were　investigated　by　X-ray　diffraction　(XRD),　optical　microscope　(OM),　scanning　electron　microscope　(SEM)　and　transmission　electron　microscope　(TEM).　XRD　results　indicated　that　the　main　phase　compositions　of　as-cast　Mg-xZn-yEr　alloys　were　the　icosahedral　quasicrystalline　phase　(I-phase)　and　α-Mg　solid　solution.　The　I-phase　mainly　distributed　in　the　dendritic　and　staccato　strips.　The　creep　tests　were　conducted　under　the　condition　of　448　K,　70　MPa　for　100　h.　As　the　addition　of　Er　increased　from　0.5　wt.%　to　1.5　wt.%,　the　total　creep　strain　decreased　from　0.962%　to　0.512%,　and　the　steady　state　creep　rate　decreased　from　1.411×10-8s-1　to　4.917×10-9s-1.　The　I-phase　had　a　tendency　to　be　bulky　and　continuous,　as　the　volume　fraction　of　Er　element　increased.　Ascribed　to　the　I-phase,　the　creep　strain　happened　and　effectively　blocked　the　movement　of　dislocations,　resulting　in　the　strengthened　as-cast　Mg-Zn-Er　alloys　and　improved　creep　resistance.　Based　on　the　investigation　of　creep　behaviors,　the　creep　mechanism　of　the　as-cast　alloy　was　mainly　grain　boundary　slipping.　©　2017　Trans　Tech　Publications,　Switzerland.