The (100) Nb∶SrTiO3/(110)ZnO heterojunction were epitaxially grown by magnetron sputtering. The typical rectification characteristic was observed before and after applying a magnetic field, but the rectifying effect was significantly enhanced after applying a magnetic field. From high and low frequency capacitance, the interface state density is increased from 3.8×1012 eV-1 cm-2 to 8.2×1012 eV-1 cm-2 after applying a magnetic field. Furthermore, the Nb∶SrTiO3/ZnO heterojunction shows strong ferromagnetic properties at the range of -1~1 T under the diamagnetic condition. Thus the magnetic field enhanced rectifying effect can come down to the increase of interface state density.