Hydrothermal process was successfully used to synthesize Fe3O4 powder using ferrous chloride (FeCl2) and diamine hydrate (H4N2.H2O) as starting materials by carefully controlling the reaction conditions. The as-prepared Fe3O4 sample was characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM), and its magnetic properties were evaluated on a vibrating sample magnetometer (VSM). The nanoscale (40 nm) Fe3O4 powder obtained at 140 degreesC for 6 h possessed a saturation magnetization of 85.8 emu/g, a little lower than that of the correspondent bulk Fe3O4 (92 emu/g). It is suggested that the well-crystallized Fe3O4 grains formed under appropriate hydrothermal conditions should be responsible for the increased saturation magnetization in nanosized Fe3O4. (C) 2003 Elsevier Science Ltd. All rights reserved.