Biofabrication of noble monometallic platinum nanoparticles (Pt-NPs) and bimetallic gold-silver nanoparticles (AuAg-NPs) using aqueous extract of Delonix regia is presented here. Antioxidant activity of biomatrix-loaded metallic nanoparticles is estimated for scavenging of two model radicals i.e., 2,2'-Azino-bis-(3-ethylbenzothiazoline-6-sulfonic acid) diammonium salt and 1,1'-Diphenyl-1-2-picrylhydrazyl. Broad spectral continuum spanning from visible to ultra-violet region (Pt-NPs: 30 min) and broad high intensity absorption peak around at 500 nm (AuAg-NPs: 10 min) in two different UV-Visible spectra confirmed the biofabrication. Nanoparticles fabricated with distorted spherical shape and crystalline face-centred-cubic geometry. Strong signal around at 2.10 keV (pure-phase platinum) and typical X-ray peaks observed at 2.20 and 3 keV suggested, co-existence and alloying interaction of Au and Ag in AuAg-NPs. zeta potential (-15.2 mV: Pt-NPs and -13.9 mV: AuAg-NPs) values suggested surface adsorption of polyphenolic compounds to provide stability. Nanoparticles exhibited pronounced antioxidant activity against free radicals through their electron/hydrogen transfer ability. (C) 2018 The Chemical Industry and Engineering Society of China, and Chemical Industry Press. All rights reserved.