Polycrystalline Fe3O4 films have been prepared by reactive sputtering at room temperature. Transmission electron microscopy (TEM) images show that the films consist of quite uniform Fe3O4 grains well separated by grain boundaries. It was found that the tunneling of spin polarized electrons across the antiferromagnetic coupled grain boundaries dominates the transport properties of the films. Magnetoresistance MR=ρ(H)-ρ(o)=ρ(0) shows linear and quadratic magnetic field dependence in the low field range when the field is applied parallel and perpendicular to film plane, which is similar to the behaviors observed in the epitaxial Fe3O4 films consisting of a large fraction of antiferromagnetic antiphase domain boundaries (APBs). At 300 K, the size of MR reaches-7.4% under a 50 kOe magnetic field, which is, by far, the largest MR observed in polycrystalline Fe3O4 films.