Due　to　their　unique　precipitation　behavior,　magnesium-rare　earth　(Mg-RE)　alloys　exhibit　excellent　mechanical　properties　and　decent　thermal　stability.　In　this　work,　a　Mg-Gd-Y-Ag-Zr　alloy　was　employed　to　investigate　the　segregation　and　interfacial　phase　formation　at　grain　boundaries　after　plastic　deformation　and　heat　treatment.　The　interfacial　phase　was　unequivocally　investigated　by　aberration-corrected　high-angle　annular　dark-filed　scanning　transmission　electron　microscopy　(HAADF-STEM)　from　three　different　crystal　directions　and　modeling,　which　reveals　a　hitherto　unknown　crystal　structure　(monoclinic:　(beta　=　139.1　degrees,　a　=　1.20　nm,　b　=　1.04　nm　and　c　=　1.59　nm).　Its　orientation　relationship　with　the　Mg　matrix　is:　[101]//[11　(2)　over　bar0](alpha),　[302)//[10　(1)　over　bar0](alpha)　and　(010)//(0001)(alpha).　Different　from　the　precipitates　in　matrix,　the　size　of　the　interfacial　phase　was　not　sensitive　to　annealing　temperature　between　250　degrees　C　and　400　degrees　C.　Transformation　of　twin　boundaries　to　coaxial　grain　boundaries　via　multiple　twinning　led　to　the　generation　of　many　high　strain　sites　along　the　boundaries,　which　promoted　the　formation　of　the　interfacial　phase.　The　interfacial　phase　was　stable　up　to　400　degrees　C,　which　was　about　100　degrees　C　higher　than　the　dissolution　temperature　of　beta＇　and　gamma　＇＇　precipitates.　(C)　2018　Acta　Materialia　Inc.　Published　by　Elsevier　Ltd.　All　rights　reserved.