Fe3O4 magnetic nanoparticles were prepared by the aqueous co-precipitation of FeCl3·6H2O and FeC12·4H2O with addition of ammonium hydroxide. The conditions for the preparation of Fe3O4 magnetic nanopar-ticles were optimized, and Fe3O4 magnetic nanoparticles obtained were characterized systematically by means of transmission electron microscope (TEM), dynamic laser scattering analyzer (DLS) and X-ray diffraction (XRD). The results revealed that the magnetic nanoparticles were cubic shaped and dispersive, with narrow size distribution and average diameter of 11.4 nm. It was found that the homogeneous variation of pH value in the solution via the control on the dropping rate of aqueous ammonia played a critical role in size distribution. The magnetic response of the product in the magnetic field was also analyzed and evaluated carefully. A 32.6 mT magnetic field which is produced by four ferromagnets was found to be sufficient to excite the dipole moments of 0.05 g Fe3O4 powder 2 cm far away from the ferromagnets. In conclusion, the Fe3O4 magnetic nanoparticles with excellent properties were competent for the magnetic carriers of targeted-drug in future application. Fe3O4 magnetic nanoparticles were prepared by the aqueous co-precipitation of FeCl3 · 6H2O and FeC12 · 4H2O with addition of ammonium hydroxide. The conditions for the preparation of Fe3O4 magnetic nanopar-ticles were optimized, and Fe3O4 magnetic nanoparticles were successfully systematically by means of transmission electron microscope (TEM), dynamic laser scattering analyzer (DLS) and X-ray diffraction (XRD). The results revealed the magnetic nanoparticles were cubic shaped and dispersive, with narrow size distribution and average diameter of 11.4 nm. that the homogeneous variation of pH value in the solution via the control on the dropping rate of of aqueous ammonia played a critical role in size distribution. A magnetic field of was 16.6 mT magnetic field was also analyzed and evaluated carefully. which is produced by four ferromagnets was found to be sufficient to excite the dipole moments of 0.05 g Fe3O4 powder 2 cm far away conclusion from the ferromagnets. In conclusion, the Fe3O4 magnetic nanoparticles with excellent properties were competent for the magnetic carriers of targeted-drug in future application.