In　recent　years,　Zr　is　widely　used　as　an　important　additive　element　in　magnesium　alloys　containing　rare　earth　(RE),　to　improve　the　mechanical　properties　of　Mg-RE　alloys　such　as　strength,　ductility,　creep　resistance　and　corrosion　resistance　property.　Heterogeneous　nucleation　mechanism　and　peritectic　reaction　mechanism　are　recognized　as　the　main　grain　refining　mechanisms.　Whereas,　during　the　solidification　process,　the　melt　wetting　angle　and　nucleation　energy　are　important　factors　which　influence　the　nucleation.　In　this　work,　the　effect　of　Zr　on　the　solidification　microstructure　of　the　Mg-Gd-Er　alloy　was　analyzed　by　using　OM　and　EBSD;　the　undercooling　of　alloy　melts　was　tested　by　using　DSC;　and　the　Mg/Zr　interface　relationship　and　interfacial　energy　were　investigated　by　using　HRTEM.　Moreover,　the　effects　of　Zr　on　the　wetting　angle　and　nucleation　activation　energy　of　the　Mg-11Gd-2Er　and　Mg-11Gd-2Er-0.4Zr　alloys　were　investigated;　the　refinement　mechanism　of　Zr　on　the　alloys　was　discussed.　The　results　indicates　that　the　addition　of　Zr　element　can　significantly　refine　the　grain,　and　the　grain　size　decreased　from　1000　mm　to　50　mu　m.　Compared　with　the　Zr-free　alloy,　the　nucleation　wetting　angle　of　the　present　alloy　melt　decreased　from　18.3　degrees　to　11.1　degrees,　and　the　activation　energy　of　nucleation　decreased　by　44.4%.　The　(1010)　plane　of　Mg　was　completely　coherent　with　the　(1100)　plane　of　Zr,　reducing　the　interfacial　energy　between　the　(1010)　Mg　and　the　(1100)　Zr.　The　grain　refinement　of　Mg-Gd-Er　alloy　was　ascribed　to　the　decrease　of　melt　wetting　angle　and　the　fully　coherent　interface　relationship　between　Mg　and　Zr.