Bio-mediated synthesis of metal oxide nanoparticles using plant extract is a promising alternative of traditional chemical synthesis. The present study reports the synthesis of ZnO nanoparticles by biological method. Highly stable and hexagonal phase ZnO nanoparticles were synthesized using Pongamia pinnata leaves extract and were characterized by XRD, UV-vis, DLS, SEM, TEM and FT-IR spectroscopy. The synthesized ZnO nanoparticles were confirmed by XRD and FTIR spectra. Morphology studies indicates spherical nature of the ZnO NPs and EDX shows the highly pure ZnO nanoparticles. The antibacterial activity of ZnO nanoparticles and ZnO nanoparticles coated cotton fabric were tested against Staphylococcus aureus (gram positive) and Escherichia coli (gram negative) organisms by agar diffusion method. Finally, the current study has clearly demonstrated that the ZnO NPs are responsible for significant higher antibacterial activities. Therefore, the study reveals an efficient, ecofriendly and simple method for the green synthesis of multifunctional ZnO NPs using green synthetic approach. (C) 2015 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.