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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: β= 139.1°, a= 1.20 nm, b= 1.04 nm and c= 1.59 nm). Its orientation relationship with the Mg matrix is:[101]//[11 2¯ 0] α,[302]//[10 1¯ 0] α and (010)//(0001) α. Different from the precipitates in matrix, the size of the interfacial phase was not sensitive to annealing temperature between 250° C and 400° C …