A facile method for synthesis of the magnetic Fe3O4 nanoparticles was introduced. Magnetic nanoparticles were prepared via co-precipitation method with (PMF) and without (AMF) 0.15 T static magnetic field. The effects of magnetic field on the properties of magnetic nanoparticles were studied by XRD, TEM, SEM, VSM and BET. The results showed that the magnetic field in the co-precipitation reaction process did not result in the phase change of the Fe3O4 nanoparticles but improved the crystallinity. The morphology of Fe3O4 nanoparticles was varied from random spherical particles to rod-like cluster structure. The VSM results indicated that the saturation magnetization value of the Fe3O4 nanoparticles was significantly improved by the magnetic field. The BET of Fe3O4 nanoparticles prepared with the magnetic field was larger than the control by 23.5%. The batch adsorption experiments of Mn(II) on the PMF and AMF Fe3O4 nanoparticles showed that the Mn(II) equilibrium capacity was increased with the pH value increased. At pH 8, the Mn(II) adsorption capacity for the PMF and AMF Fe3O4 was reached at 36.81 and 28.36 mg.g(-1), respectively. The pseudo-second-order model fitted better the kinetic models and the Freundlich model fitted isotherm model well for both PMF and AMF Fe3O4. The results suggested that magnetic nanoparticles prepared by the magnetic field presented a fairly good potential as an adsorbent for an efficient removal of Mn(II) from aqueous solution. (C) 2016 The Chemical Industry and Engineering Society of China, and Chemical Industry Press. All rights reserved.