Real-time　location　system　(RLS)　based　on　RFID　is　an　effective　indoor　positioning　system.　The　battery-free　and　low　cost　UHF　passive　tags　can　be　attached　on　almost　any　objects,　which　are　recognized　as　the　best　medium　to　achieve　high　precision　ranging　and　positioning　for　large-scale　objects.　This　paper　proposes　an　indoor　range　measurement　based　on　multifrequency　phase　difference　of　arrival　(MF-PDoA)　using　UHF　RFID　passive　tags　and　discusses　the　measurement　principle,　experiment　implementation,　and　results　evaluation　in　detail.　After　a　theoretical　overview　of　MF-PDoA　range　measurement　principle,　it　introduces　an　experimental　prototype　under　EPC　C1G2　standard　for　range　measurements.　Both　our　prototype　and　a　commercial　off-the-shelf　RFID　reader　have　been　used　to　verify　the　measurement　method.　We　propose　a　Kalman　filter　and　weighting　method　to　process　the　measuring　data.　Experiment　results　indicate　that,　in　a　real　environment,　this　method　can　effectively　improve　the　ranging　accuracy,　which　lays　a　foundation　to　extend　the　proposed　measurement　into　two　to　three　dimensions　indoor　object　positioning.