Green synthesis process has been developed to produce MgO nanoparticles using wet chemical method. In this process, Madhuca longifolia (M. longifolia) flower extracts has been used as a reducing and stabilizing agent and magnesium nitrate hexahydrate is used as a metal precursor. This study exhibits the effect of varied concentrations of metal precursors and flower extracts on the shape and size of the MgO nanoparticles. FTIR, UV vis spectroscopy, SEM, EDS and HRTEM analysis have been carried out to identify the functional groups involved in the reduction process, confirmation of MgO nanoparticles, morphological and elemental analysis of the MgO nanoparticles respectively. The results confirmed the presence of spherical MgO nanoparticles decorated over the surface of MgO microclusters. It is observed that the varying concentration of flower extracts and precursor have a profound effect on morphology of the nanoparticles resulting in the decrease in cluster size with the increase in extract concentration. The nanoparticles were also tested for the presence of antibacterial activity against the bacterial species of Escherichia coli and Staphylococcus aureus. © 2022 Taylor & Francis Group, LLC.