At room temperature, polycrystalline Fe3O4 films with thicknesses ranging from 5 to 1120 nm have been prepared by reactive sputtering. Transmission electron microscopy image shows that uniform Fe3O4 grains are well separated by grain boundaries and their size decreases with the film thickness. The changes of resistivity as a function of temperature reveal a grain boundary dominated electron tunneling transport mechanism. The magnetoresistance MR=ρH-ρo=ρ0 measured at room temperature for the films with thickness larger than 200nm is～7.4% in 46 kOe magnetic field, which is one of the largest value have ever been reported for Fe3O4 films under the same conditions. As the thickness reduced from 80 to 5nm, MR decreases from～6.5% to～1.1% due to magnetization reduction and the enhanced spin-flip scattering at the film surface.