Mg-Gd　based　alloys　are　typical　high　strength　magnesium　alloys　owing　to　their　outstanding　age　hardening　behavior.　The　mechanical　strength　of　aged　Mg-Gd　alloys　is　enhanced　by　high　density　prismatic-shaped　precipitates　in　Mg　matrix.　Moreover,　the　addition　of　Ag　has　been　found　to　enhance　the　strength　of　Mg-Gd　based　alloys　further　by　forming　a　plate-shaped　precipitate,　named　as　gamma　＇＇,　on　basal　planes　of　Mg.　However,　the　structure　of　this　precipitate　is　not　well　understood　due　to　the　complex　alloying　system.　In　this　work,　the　atomic　structure　of　gamma　＇＇　phase　is　investigated　using　atomic-resolution　high-angle　annular　dark　field　scanning　transmission　electron　microscopy　(HAADF-STEM).　Based　on　the　accurate　atomic　structure　from　three　independent　directions,　e.g.　in　[0001](Mg),　[110](Mg)　and　[010](Mg)　zone　axes,　the　unit　cell　of　gamma　＇＇　is　well　established.　The　gamma　＇＇　phase　has　a　hexagonal　structure,　and　its　lattice　parameters　are　a　=　0.55　nm　and　c　=　0.42　nm.　The　orientation　relationship　between　gamma　＇＇　phase　and　Mg　matrix　is　(0001)(gamma　＇＇)//(0001)(Mg)　and　(gamma　＇＇)//(Mg).