It is very important to control the substructure of a membrane prepared by the phase inversion process. This article reports a novel method to control the substructure of ultrafiltration (UF) membrane by the combined effect of a magnetic filler and a parallel magnetic field. A series of polysulfone (PSF) ferrosoferric oxide (Fe3O4) UF membranes with different amounts of Fe3O4 were prepared in a parallel magnetic field from suspensions, using the phase inversion process. The suspensions consisted of PSF, N,N-dimethylacetamide, poly(vinylpyrrolidone), and Fe3O4. Magnetic Fe3O4 particles in a casting solution are expected to arrange along the direction of a magnetic field during the membrane formation. This kind of oriented arrangement can gradually change the cross-sectional microstructure of a membrane from normal finger-like macrovoids perpendicular to the membrane surface into macrovoids parallel to the membrane surface, with increasing Fe3O4 content. As a result, a novel membrane with "lamellar macrovoids" (parallel to the membrane surface) in the sublayer was prepared as the Fe3O4 content reached 70 wt %. Furthermore, the membrane with 70 wt % Fe3O4 not only had the best flux and rejection but also had a good antipressure ability. The formation mechanism of novel microstructure of the sublayer in the UF membrane is also discussed. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 117: 1960-1968, 2010