This study describes biosynthesis of Zinc oxide nanoparticles (ZnO NPs) using agro-waste durva grass aqueous extract without uses of any hazardous chemicals and stabilizing agents. Surface plasmon resonance (SPR) band of the synthesized ZnO NPs confirmed its conversion from its metal salt to NPs by UV-vis spectroscopy. Characterization of functional groups in durva grass aqueous extract and ZnO NPs surfaces were identified by Fourier transform infrared spectroscopy (FTIR), purity and crystallinity by power X-ray diffraction analysis (XRD), size and morphology by transmission electron microscopy analysis (TEM), elemental composition by Energy Dispersive X-ray spectroscopy (EDAX), stability by thermogravimetric analysis (TGA) and dynamic light scattering (DLS) study. The synthesized ZnO NPs was highly stable with average particle size of 17 nm. The photocatalytic activity of ZnO NPs was studied under UV irradiation to methylene blue (MB) and methyl orange (MO) degradation (10 mg l-1 each). 98.1% of MB was degraded within 60 min, while 97.5% of MO within 100 min. In addition, antioxidant activity of ZnO NPs showed 91.8% free radical scavenging capacity at the dose of 250 mg l-1 which implies its importance in biomedical applications. © 2020 The Author(s). Published by IOP Publishing Ltd.