Rectifying properties, with and without magnetic field, of a high quality Fe3O4/SrTiO3:Nb Schottky diode have been experimentally studied. The junction exhibits an excellent rectifying behavior both below and above the Verwey temperature (TV) of Fe3O4. Magnetic field has a weak but visible effect on the transport process of the junction, producing a negative magnetoresistance for T<TV and a positive magnetoresistance for T>TV. Based on an analysis of the current-voltage characteristics, the spin polarization of Fe3O4 has been deduced. It is a strong function of temperature, varying between −78% and 18%. © 2007 American Institute of Physics.