Iron nanoparticle (INP) is one of metal nanoparticles that have a wide application such as adsorbent, catalyst, drug delivery, indicator in health analysis, etc. Usually, INP is synthesized by physical or chemical means. Both methods have their respective advantages and disadvantages. However, with the increased awareness of green chemistry and technology, green synthesis of INP, using various plant extracts, has become a huge interest. In this study, we utilized crude extract of leucaena seed

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... Leucaena leucocephala) as green solvent to synthesize INP. The synthesis was done by mixing leucaena crude extract with 0.1 M FeCl3 with volume ratio 1:1. The mixture turned to a dark violet color, indicating a complex formation between Fe 3+ ions and organic compounds in the extract. After pH adjustment to 6, iron precipitate was separated, dried, and calcined at 500°C for 3h. The obtained INP was characterized using Scanning Electron Microscopy -Energy Dispersive X-Ray Spectroscopy (SEM-EDS) X-ray Diffraction (XRD), and Transmission Electron Microscope (TEM). It was found that the INP was in form of agglomerate, and after calcination smaller agglomerate was observed without any presence of organic compounds. The calcination also changed the phase of INP to be more crystalline with hematite phase as dominant structure. Further application as Fenton-like catalyst showed increase of methylene blue degradation kinetics to 5 times compared with one without catalyst. A 32% methylene blue removal was obtained with initial methylene blue concentration of 10 ppm, INP concentration 40 mg/L after 180 min reaction.