Polycrystalline Fe3O4/Al bilayers with Al underlayers (ULs) of different thicknesses were fabricated using facing-target reactive sputtering. Structure analyses revealed that Al ULs improve the crystallinity and reduce the thickness of the Fe3O4 amorphous initial layer. Room-temperature magnetization of the Fe3O4 films with 30 nm thick Al ULs is ~450 emu cm−3 at 50 kOe fields, which is much larger than that of the polycrystalline Fe3O4 monolayer deposited under the same conditions. The field-cooled hysteresis loop of Fe3O4 films on the 30 nm thick Al layer shows a 50 Oe exchange bias field at 5 K, much smaller than 256 Oe detected in polycrystalline Fe3O4 monolayer and the exchange bias fields in the samples with and without 30 nm Al UL decrease with the increasing temperature. The enhancement of the magnetization and the sharp decrease in the exchange bias field observed in the Fe3O4 films with Al ULs can be attributed to the decoupling of the antiferromagnetic domains and the thickness reduction of the amorphous Fe–O layers between the polycrystalline Fe3O4 films and the Al ULs.