MgO　thin　films　of　different　thicknesses　were　prepared　on　polished　Ag-3wt%Mg　alloy　by　an　activation　process　at　500-650　degrees　C　under　oxygen　pressures　of　0.5-10.0　Pa.　The　influence　of　the　thickness　of　MgO　film　on　the　secondary　electron　yield　of　the　Ag-3wt%Mg　alloy　is　investigated.　The　alloy　with　an　MgO　film　of　moderate　thickness　of　65　+/-　1　nm　shows　the　highest　secondary　electron　yield　of　10.　Oxygen　pressure　below　10.0　Pa　is　crucial　for　the　formation　of　intact　MgO　film　on　the　top　surface.　Computational　simulation　analysis　reveals　that　the　maximum　yield　occurs　when　the　penetration　depth　of　primary　electrons　is　13-18　nm　less　than　the　thickness　of　MgO　film.　Electron　supply　of　this　extra　MgO　film　is　allowed　by　an　electron　tunneling　mechanism　under　certain　bias　voltage.　The　replenishment　of　new　electrons　to　the　electron　deficient　surface　is　crucial　for　obtaining　optimized　secondary　electron　yields　for　Ag-Mg　alloy　cathodes.　(C)　2016　Elsevier　Ltd.　All　rights　reserved.