In this study, leaf extract from Hageniaabyssinica (kosso) was added to hexa hydrate ferric chloride (FeCl3.6H2O) to transform it into iron oxide nanoparticles (Fe3O4NPs). FTIR, XRD, and UV-visible measurements were employed to validate the generation of the iron oxide nanoparticles and to thoroughly describe them. The presence of active flavonoids, phenolic groups, alkaloids, and tannins in the biomass of the kosso leaf extract reduction was ascertained using FT-IR Spectroscopy and qualitative screening methods (searching for color changes).The green synthesis iron oxide nanoparticles were characterized by UV-Visible, FT-IR, and XRD spectroscopes. The reduction process was simple to handle and was monitored by UV-Visible spectroscopy that showed surface plasmon resonance (SPR) of the IONPs at 300 nm. This outcome unequivocally demonstrated that Fe3O4NPs were formed. The size and structure of the crystal were assessed using X-ray diffraction. The average particle size of Fe3O4 NPs12.21nm using the line width of the plane, refraction peak using the Scherrer’s equation. The antibacterial properties of green produced iron oxide nanoparticles against Gram +ve and Gram -ve bacteria were assessed. Iron oxide nanoparticles showed the least amount of efficacy against Staphylococcus aureus and the greatest zone of inhibition against Escherichia coli.