Ultrafiltration is widely used in waste water treatment and has become more crucial with increasing concerns in the living environment. Here we demonstrate a facile method to prepare 10 nm-diameter cellulose acetate (CA) nanofiber dispersions from very dilute CA solutions via freeze-extraction, and further fabricate nanofibrous composite membranes for ultrafiltration. The nanofibrous composite membranes are fabricated by directly filtering the dispersions on a CA microfiltration membrane (support), on which an ultrathin free-standing nanofibrous layer is formed. This layer, acting as a separation layer, has a uniform porous structure and ultrahigh porosity of up to 71%. The as-prepared membranes present ultrahigh water permeability and high efficient separation performance for ultrafiltration. The membrane with a 458 nm-thick nanofibrous layer has ferritin rejection of 90.7% and water flux of 3540 I m(-2) h(-1) bar that is almost 10 times greater than that of most commercial membranes. These newly developed CA nanolibrous composite membranes have a great potential application in various separation processes. (C) 2013 Elsevier B.V. All rights reserved