More　and　more　objects　can　be　identified　and　sensed　with　RFID　tags.　Existing　schemes　for　2D　indoor　localization　have　achieved　impressing　accuracy.　In　this　paper　we　propose　an　accurate　3D　localization　scheme　for　objects.　Our　scheme　leverages　spatial　domain　phase　difference　to　estimate　the　height　of　objects　which　is　inspired　by　the　phase-based　Interferometric　Synthetic　Aperture　Radar　(InSAR)　height　determination　theory.　We　further　leverage　a　density-based　spatial　clustering　method　to　choose　the　most　likely　position　and　show　that　it　improves　the　accuracy.　Our　localization　method　does　not　need　any　reference　tags.　Only　one　antenna　is　required　to　move　in　a　known　way　in　order　to　construct　the　synthetic　arrays　to　implement　the　locating　system.　We　present　experimental　results　from　an　indoor　office　environment　with　EPC　C1G2　passive　tags　and　a　COTS　RFID　reader.　Our　3D　experiments　demonstrate　a　spatial　median　error　of　0.24　m.　This　novel　3D　localization　scheme　is　a　simple,　yet　promising,　solution.　We　believe　that　it　is　especially　applicable　for　both　portable　readers　and　transport　vehicles.　©　2016　IEEE.