The present work proposed a membrane processing for size separation of polydispersed nanosized ferric oxide hydrates, where desalination was first achieved with polyvinylidene fluoride (PVDF) ultrafiltration (UF) membrane of 100 kDa molecular weight cut-off (MWCO), and fractionation was then realized by using cellulose acetate (CA) microfiltration (MF) membrane of 5 mu m pore size. X-ray diffraction (XRD), transmission electron microscope (TEM) and scanning electron microscope (SEM) were employed to examine size and morphology of Fe2O3 nanoparticles before and after fractionation. Specific surface areas were also determined by using the Brunauer-Emmett-Teller (BET) method. Before fractionation, the mean nanoparticle size of the feed was within 45.2-49.8 nm. After fractionation, the permeate and the retentate were found to have a smaller size (36.7-39.8 nm) and a larger size (52.4-56.9 nm), respectively. Furthermore, as compared with the feed, a smaller disperse coefficient was found for both permeate and retentate. These results demonstrated that membrane separation technique was an efficient way to achieve fractionation of Fe2O3 nanoparticles. (C) 2008 Elsevier B.V. All rights reserved.