Highly　transparent　MgO　ceramic　was　prepared　by　hot　isostatic　pressing　(HIP)　technique　of　vacuum-sintered　MgO　compact　using　CaO　as　the　sintering　additive.　Densification　process　of　MgO　ceramic　is　significantly　enhanced　by　doping　0.5　at%　CaO　at　the　vacuum-sintering　temperature　of　1100–1350　°C.　By　subsequent　HIP　sintering　at　1650　°C,　the　relative　density　of　0.5%　CaO-doped　MgO　ceramic　vacuum-sintered　at　1350　°C　increases　from　94.67%　to　99.97%.　It　attains　high　in-line　transmittance　of　70.78%　at　600　nm　and　87.59%　at　6000　nm,　respectively.　Meanwhile,　the　thermal　conductivity　reaches　53.2　W/(m∙K),　demonstrating　that　CaO-doped　MgO　is　a　promising　candidate　material　as　the　secondary　phase　of　phosphor　composite　ceramics.　Herein,　0.5%　CaO-doped　MgO-YGAG:Ce　(Y2.82Gd0.18Al5O12:Ce)　composite　ceramic　with　stable　binary-phase　structure,　high　relative　density　of　99.1%　and　thermal　conductivity　of　32.6　W/(m∙K)　was　synthesized　by　vacuum-sintering　at　1350　°C　and　the　luminescence　performance　under　blue　laser　excitation　was　evaluated.　©　2023　Elsevier　Ltd　and　Techna　Group　S.r.l.