Ultrafiltration of an aqueous solution of carboxymethyl cellulose (CMC) was carried out in a tubular-membrane model made of ZrO2 /carbon. Water was forced through the macroporous membrane as the permeate, while CMC was concentrated and recovered as the retentate. Correlation equations for calculating the permeate flux of membrane ultrafiltration were derived based on the resistance-in-series model. Correlation results were confirmed by the experimental data. Experimental results showed that the permeate flux increases as transmembrane pressure or fluid velocity increases, but decreases when feed concentration increases. Because membrane ultrafiltration is a pressure-driven process, high cross-flow velocity enhances the mass transfer coefficient of the solute and high solution concentration increases the thickness of the concentration polarization layer.