In the present study, ZnO nanoparticles doped with 3%, 5% and 7% of cobalt have been synthesized by green method using leaf extract. The obtained nanopowder was characterised by XRD, FTIR, XPS, HRTEM, SAED, SEM, EDAX and UV-Visible spectroscopy techniques. XRD patterns confirm the formation of pure and Co doped ZnO nanoparticles with a hexagonal wurtzite structure with high phase purity. FTIR spectra indicate the stretching vibration of Zn-O at 495 cm. The incorporation of Co ions into the ZnO lattice was identified by XPS analysis. EDX spectra confirm the existence of Co, Zn and O elements. The SEM and HRTEM micrographs show morphology of nanoparticles. The optical study specifies a decrease in energy band gap with an increase in Co-doping concentration. The photocatalytic performance of ZnO and ZnCoO has been examined for the degradation of methylene blue (MB) under sunlight irradiation. The antimicrobial activity of synthesized nanoparticles against , , bacterial strains and fungal strains as investigated. The ZnCoO nanoparticles exhibit good antioxidant properties. Moreover, the cytotoxicity of ZnO nanoparticles was evaluated against L929 normal fibroblast cells. So, this work suggests that leaf extract mediated pure and Co-doped ZnO nanoparticles are a potential candidate for biomedical and photocatalytic applications.