Antibacterial activity of nanoparticles (NPs) and nanocomposites (NCs) has received wide spread attention in biomedical applications. In this direction, the authors prepared zinc oxide (ZnO), iron oxide (Fe3O4), and their composite including reduced graphene oxide (rGO) by hydrothermal method. The structural and microstructural properties of the synthesised NPs and NCs were investigated by XRD, FT-IR, UV-Vis, TGA, and TEM analysis. PEG-coated ZnO and Fe3O4 form in hexagonal wurtzite and inverse spinel structures, respectively. ZnO forms in rod-shaped (aspect ratio of ∼3) morphology, whereas well-dispersed spherical-shaped morphology of ∼10 nm is observed in Fe3O4 NPs. The ZnO/Fe3O4 composite possesses a homogeneous distribution of above two phases and shows a very good colloidal stability in aqueous solvent. These synthesised particles exhibited varying antibacterial activity against gram-positive strain Staphylococcus aureus (S. aureus) and gram-negative strain Escherichia coli (E. coli). The nanocomposite exhibits a better cidal effect on E. coli when compared to S. aureus when treated with 1 mg/ml concentration. Further, the addition of rGO has intensified the anti-bacterial effect to a much higher extent due to synergistic influence of individual components. © The Institution of Engineering and Technology 2019.