Iron　nanoparticles　were　first　green　synthesized　using　polyphenols　extracted　from　Garcinia　mangostana　L.　pericarp　(GM-Fe　NPs)　and　used　as　a　catalyst　to　decolorize　Reactive　Brilliant　Blue　KN-R　(RBB,　anthraquinone　dye)　in　the　Fenton-like　oxidation　system.　Polyphenols　were　used　as　reducing　and　capping　agents　in　the　synthesis　of　GM-Fe　NPs,　which　was　demonstrated　by　scanning　electron　microscope　(SEM),　X-ray　energy-dispersive　spectrometer　(EDS),　X-ray　diffraction　(XRD),　Fourier　transform　infrared　spectroscopy　(FTIR)　of　GM-Fe　NPs　and　the　variations　of　total　polyphenols　during　synthesis.　The　XRD　patterns　suggested　that　the　synthesized　GM-Fe　NPs　were　mainly　amorphous　in　nature.　The　effects　of　extract　to　FeC13　ratio,　dosage　of　GM-Fe　NPs　and　initial　dye　concentration　on　the　RBB　decolorization　were　also　investigated.　The　optimal　ratio　of　extract　to　FeCl3　for　the　synthesis　was　5　:　1.　The　decolorization　efficiency　of　RBB　increased　with　the　increasing　GM-Fe　NPs　dosage.　With　the　initial　RBB　concentration　ranging　from　250　to　750　mg/L,　the　decolorization　efficiency　can　reach　more　than　97.83　%　after　180　min.　The　combined　first-order　kinetic　model　can　well　explain　the　RBB　degradation.　UV-vis　spectra　showed　that　color　groups　in　the　visible　region　had　been　degraded　and　no　new　peaks　appeared.　The　green　synthesized　GM-Fe　NPs　could　be　employed　as　a　low-cost,　efficient　and　alternative　catalyst　in　Fenton-like　oxidation　system　for　RBB　removal.