Porosity　of　top　coating　(TC)　in　thermal　barrier　coatings　(TBCs)　is　one　of　the　main　factors　affecting　the　thermal　conductivity　and　causing　failure　of　TBCs.　To　ensure　the　quality　of　the　TBCs,　the　evaluation　of　TC　porosity　was　carried　out　using　a　microwave　non　destructive　technique.　The　reflection　coefficient　at　the　interface　of　TC　and　waveguide　probe　was　studied　with　regard　to　the　TC　porosity　using　computer　simulation　technology-microwave　studio　(CST-MWS).　The　results　showed　that　the　greater　the　porosity　of　the　TC　the　larger　the　phase　difference　when　the　microwave　was　operated　at　the　sensitive　frequency.　And　the　sensitive　frequency　is　different　when　TC　thickness　changes.　Besides,　a　network　analyzer　was　used　to　measure　the　reflection　coefficient　phase　which　can　be　used　in　the　evaluation　of　the　TC　porosity.　The　results　showed　that　the　phase　difference　increases　linearly　with　increasing　porosity　of　TC　when　the　network　analyzer　operating　over　the　frequency　range　of　30　GHz　to　40　GHz.　Therefore,　the　porosity　of　TC　can　be　calculated　by　measuring　the　phase　of　the　reflection　coefficient.　The　experimental　results　are　consistent　with　the　theory　simulation　ones.　Crown　Copyright　(C)　2013　Published　by　Elsevier　Ltd.　All　rights　reserved.