The green plant-mediated synthesis of iron nanoparticles (GPS-Fe) nanoparticles (NPs) has become more and more popular. In this study, fresh rosemary leaf aqueous extracts that decreased aqueous silver nitrate were used to demonstrate the synthesis of FeNPs. This procedure allowed for the synthesis of NPs that were examined using a different of analytical techniques, including transmission electron microscopy (SEM), energy-dispersive X-ray spectroscopy, FTIR, ultraviolet-visible (UV-Vis) spectrophotometry, and studies on dynamic light scattering. According to ocular observation and UV-Vis spectra, fresh leaf extract treatment changed the color of aqueous ferric nitrate to a bluish hue. Additionally, the TEM analysis revealed that the synthesized NPs were evenly dispersed and had average sizes less than 65 nm. Meanwhile, the bioactivity test shows that the NPs had an inhibitory effect on a number of pathogenic bacteria and caused a high rate of mortality in three stages of house fly larvae.