In this study, a new class of high performance thin film nanocomposite (TFNC) ultrafiltration membrane based on a polyacrylonitrile (PAN) nanofibrous substrate coupled with a thin hydrophilic nanocomposite barrier layer was fabricated by electrospinning technique combined with solution treatment method, and was used as an ultrafiltration media to separate an oil/water emulsion. The hydrophilic nanocomposite barrier layer was composed of crosslinked poly(vinyl alcohol) (PVA) thin layer incorporating surface oxidized multi-walled carbon nanotubes (MWNTs), and was prepared by immersing electrospun PVA-MWNT/PAN nanofibrous double-layer mats into optimized water/acetone solution and then chemically crosslinked by glutaraldehyde in water/acetone solution. The electrospun PVA-MWNT nanofiber top layer would be swollen to merge imperceptibly into an integrated barrier film on the supporting PAN layer. The variation of the free volume of PVA barrier layer with different MWNTs contents was investigated by positron annihilation lifetime spectroscopy (PALS). The PALS results proved that the free volume of PVA-MWNT/PAN TFNC membranes increased markedly with the increase of MWNTs concentration in the PVA layer, and the filtration evaluation also confirmed that the incorporation of MWNTs into PVA barrier layer could improve the water flux significantly, which indicated that more effective water channels were generated in the nanocomposite barrier layer by the incorporation of MWNTs into PVA barrier layer. The PVA-MWNT/PAN TFNC (10 wt.% MWNT) membrane showed very high water flux (270.1 l/m(2) h) with high rejection rate (99.5%) even at very low feeding pressure (0.1 MPa). In addition, the PVA-MWNT/PAN composite membranes showed very good overall mechanical properties. (C) 2013 Elsevier B.V. All rights reserved.