Thermal　barrier　coatings　(TBCs)　have　been　widely　used　in　gas　turbine　engines　in　order　to　protect　the　substrate　metal　alloy　against　high　temperature　and　to　enhance　turbine　efficiency.　Thermal　effects　in　the　work　process　will　produce　a　certain　stress　to　induce　the　metal　substrate　surface　defects　such　as　cracks,　which　affect　thermal　conductivity　of　TBCs.　Therefore,　it　is　necessary　to　detect　the　cracks　in　order　to　ensure　the　quality　of　TBCs.　In　this　paper,　an　approach　for　microwave　non-destructive　evaluation　of　surface　crack　on　the　substrate　under　thermal　barrier　coatings　will　be　presented.　The　relationship　between　reflection　coefficient　and　defects　of　TBCs　were　studied,　based　on　the　propagation　feature　of　microwave　isonified　through　multi-layer.　A　network　analyzer　operated　in　the　frequency　range　of　26.5　GHz　to　40　GHz　was　used　to　measure　the　reflection　coefficient.　Scanning　images　were　obtained　by　using　phase　difference　and　amplitude　at　the　optimum　detection　frequency.　The　results　indicate　that　the　amplitude,　and　the　phase　difference　of　the　reflection　coefficient,　can　represent　the　defects　under　TBCs　using　microwave　non-destruction　evaluation　when　the　stand-off　distance　is　suitable.　The　experimental　setup　that　validates　the　microwave　non-destructive　inspection　of　TBCs　is　presented,　and　the　feasibility　to　evaluate　the　cracks　on　the　substrate　surface　using　microwave　non-destructive　evaluation　is　verified　in　this　paper.