Magnetite (Fe3O4) nanopowder can be prepared by one-pot solvothermal reaction of Fe-urea complex ([Fe (CON2H4)(6)](NO3)(3)) in ethanol. The result of X-ray powder diffraction (XRD) suggests the formation of Fe3O4 whose lattice constant is 8.385 angstrom. The formation of Fe3O4 is further confirmed from X-ray photoelectron spectroscopy (XPS) measurements. Transmission electron micrograph (TEM) observations show that Fe3O4 particles are nearly spherical in shape. The crystallite size of Fe3O4 can be controlled from 9.7 to 20.5 nm by varying the solvothermal reaction time. Room temperature magnetization hysteresis curves exhibit almost immeasurable values of coercivity and remanence, suggesting that the Fe3O4 nanopowder possesses superparamagnetic characteristics. The saturation magnetization (Ms) increases from 32.6 to 42.9 emu/g when the reaction time increases from 10 to 50 h. (C) 2009 Elsevier B.V. All rights reserved.