Fe3O4 thin films have been fabricated on glass substrate by the facing-target sputtering technique and their field-modulated voltage-current behavior was investigated. The nonlinear dependence of voltage on current density displays a switching from high-resistivity to low-resistivity states above a threshold current density. The low-resistivity state is very sensitive to the applied magnetic field, and a large negative magnetoresistance of ∼-27% is observed at 300 Oe under a high current density of 100 A cm-2 at room temperature. Furthermore, the dependence of the magnetoresistance on the magnetic field reveals a good linear relationship. The observed results seem to favor a picture of spin-polarized intergrain tunneling through the grain boundaries.