The Fe 3 O 4-PEG magnetic nanoparticles (NPs) were prepared by hydrothermal method at different concentrations (FeCl 3 •6H 2 O 0.75 mg/mL and FeCl 3 •6H 2 O 1.5 mg/mL) and subsequently surface-functionalized coating with polyethylene glycol (PEG), the successful coating of PEG molecules on the surface of Fe 3 O 4. These magnetic NPs exhibited good dispersibility and dissolvability in physiological condition. The obtained magnetic nanoparticles were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier transform infrared (FTIR) spectroscopy, thermogravimetry (TG) and vibrating sample magnetometer (VSM). The antibacterial activity of Fe 3 O 4-PEG magnetic nanoparticles (MNPs) was studied against two bacterial strains: Gram-positive Staphylococcus and Gram-negative Escherichia coli aureus. The modified MNPs had a significant effect is more on S. aureus and less on E. coli. The results showed that polyethylene glycol-functionalized magnetic (Fe 3 O 4) NPs as a novel DNA-mediated antibacterial agent.