Mixing　enthalpy　of　Al-Er　binary　alloys　system　has　been　calculated　based　on　Miedema　model,　in　which　a　new　interaction　parameters　L-i　proposed　by　George　Kaptay　was　considered:　L-i=h(0i).exp(-T/tau(0i)).　which　are　used　to　indirectly　calculate　the　activity　of　Er　in　Al-Er　alloys　and　some　other　thermodynamic　properties,　such　as　excess　Gibbs　energy　and　excess　entropy　at　1073K.　Firstly　the　interaction　parameter　between　Al　and　Er　is　worked　out,　then　formation　ability　of　alloy　solution　is　analyzed.　The　results　showed　that　interaction　parameter　vs　temperature　can　be　taken　as　the　form　of　LAl-Er=-240.29　exp(-T/2000)　kJ/mol,　which　indicates　that　Al　and　Er　atoms　have　a　great　tendency　to　form　intermetallic　compound　of　Al3Er　during　solidification.　The　Er　concentration　shows　a　negative　deviation　from　Rault＇s　law,　mixing　enthalpy,　excess　Gibbs　energy　and　excess　entropy　are　all　negative　in　the　whole　range　of　concentration,　which　reveals　that　Al　and　Er　Could　dissolve　each　other　up　the　liquidus　line.