The biogenic synthesis of nanosize powder is of great interest due to its multifunctional applications and easy and eco-friendly process. The biosynthesis of nanosize iron particles using Amaranthus dubius leaf aqueous extracts that are reduced from ferric chloride was investigated. A. dubius aqueous leaf extraction process parameters were optimized based on the antioxidant capacity for attaining efficient extract. The leaf extract contains a rich source of amaranthine and phenolic compounds that are used as reducing agents. The effects of different process parameters like pH, temperature, concentration of leaf extract and time on the nanoparticle synthesis were examined. The synthesised nanoparticle size, morphology, crystallinity, composition, and stability were investigated. Results showed that A. dubius extract mediated nanoparticles are in spherical shape with a cubic phase structure and a diameter range from 43 to 220 nm with less aggregation. A. dubius and sodium borohydride mediated iron nanopartides for decolouration efficiency against methylene orange and antioxidant against 1, 1-diphenyl-2-picrylhydrazyl were studied. (C) 2015 Elsevier B.V. All rights reserved.