Magnetite nanoparticles have been synthesized under magnetic fields by the method of co-precipitation. The as-prepared samples were characterized by X-ray diffraction and transmission electron microscopy, and their magnetic properties were evaluated on a vibrating sample magnetometer. It is shown that the Fe3O4 samples produced by co-precipitation method under a magnetic field of 1 T have much higher saturation magnetization (15.3 emu/g) than that without magnetic field (6.59 emu/g). It is suggested that the well-crystallized Fe3O4 grains formed under magnetic fields should be responsible for the improved magnetic properties of nanosized Fe3O4 powders. (C) 2004 Elsevier B.V. All rights reserved.