BACKGROUND: Magnetic nanoparticles (MNPs) loaded by various active compounds can be used for targeted drug delivery. Objectives: In the present study, the FeO magnetic nanoparticles that contained gentamicin were prepared and their antibacterial activities were studied. MATERIALS AND METHODS: MNPs containing gentamicin (G@SA-MNPs) were prepared using sodium alginate (SA) as a surface modifier. After and before coating, the prepared MNPs were characterized using transmission electron microscopy (TEM), X-ray diffraction spectroscopy (XRD), Fourier transform infrared spectroscopy (FTIR), and vibrating sample magnetometer (VSM). Finally, the antibacterial effect of the MNPs was investigated by a conventional serial agar dilution method. RESULTS: Particle size distribution analysis showed that the size of MNPs, before and after coating, was in the range of 1-18 nm and 12-40 nm, respectively. The magnetization curve of G@SA-MNPs (with saturation magnetization of 27.9 emu.g) confirmed ferromagnetic property. Loading gentamicin on the surface of MNPs was qualitatively verified by FTIR spectrum. Quantitative analysis measurements indicated the gentamicin loading on SA-MNPs as 56.7 ± 5.4%. The measured MICs of G@SA-MNPs for (PTTC 1574) was 1.28 µg.mL. The sub-MIC (0.64 µg.mL) concentration of G@SA-MNPs in nutrient broth could successfully inhibit the growth of for 14 hours. CONCLUSIONS: Loading gentamicin on the SA-MNPs exhibited reasonable antibacterial effects against .