A　gradient-refractive-index　radar　absorbing　structure(GRINRAS)　for　stealth　aircraft　is　proposed　in　this　paper.　It　is　a　layered　shell　composed　of　a　gradient-refractive-index　(GRIN)　refractor,　an　absorber　and　a　reflector.　The　GRIN　refractor　consists　of　the　isotropic　non-resonant　woodpile　metamaterials,　which　was　designed　according　to　the　permittivity　distribution　equation　of　the　electromagnetic(EM)　black　hole　to　bend　and　match　the　waves　from　the　air　to　the　radar　absorbing　structure.　The　absorber　dissipates　the　EM　waves　and　transforms　them　into　the　heat　energy.　The　reflector　reflects　the　EM　waves　to　the　absorber　and　prevents　them　from　entering　the　inner　space　of　the　structure.　A　ring　and　a　board　GRINRAS　were　designed,　and　then　fabricated　by　the　three-dimensional　(3D)　printing　process　of　stereolithography　(SL).　Simulation　and　experimental　results　show　that　the　GRINRASes　can　control　the　wave　propagation　and　their　absorption　capacity　is　better　than　-10　dB　in　the　broadband　of　12-18　GHz.　The　GRIN　refractor　and　the　absorber　have　almost　equal　contributions　to　the　absorption　capability.　This　is　the　first　radar　absorbing　composite　structure　composed　of　the　gradient　3D　metamaterials.　It　demonstrates　the　feasibility　of　using　the　metamaterials　and　3D　printing　technology　in　the　innovative　radar　absorbing　structure　(RAS)　of　the　stealth　aircraft.　(C)　2017　Elsevier　Ltd.　All　rights　reserved.