The present study intended for the chemical synthesis of iron oxide nanoparticles (IO-NPs) followed by characterization and evaluation of antibacterial activity after treating with Argemone mexicana L. leaf extract. The formation of IO-NPs was confirmed by the colour change and further examined by UV-vis spectroscopy. The morphology was characterized by using SEM and TEM, which showed spherical particles of uniform size ranged between 10 and 30 nm and the crystallites were determined through XRD. The peaks in XRD pattern are in good agreement with that of face-centered cubic form of iron oxide nanoparticles. FT-IR spectroscopy confirmed the attachment of bioactive molecules of plant on the IO-NPs surfaces. Furthermore, the antibacterial efficacy of IO-NPs, plant extract and IO-NPs treated with plant extract was screened against Escherichia coli MTCC 443, Proteus mirabilis MTCC 425 and Bacillus subtilis MTCC 441. The results showed a noteworthy inhibition on P. mirabilis and E. coli with IO-NPs treated plant extract. This outcome may pave a way for using the magnetic nanoparticles as a drug carrier system to cure bacterial diseases. (c) 2013 Elsevier Ltd. All rights reserved.