We have demonstrated, for the first time, rectangular membranes with multiple hollow holes, a hybrid of flat sheet membranes and hollow fibers, have been successfully produced for ultrafiltration applications. The spinning process was carried out with a specially designed spinneret containing a rectangular slit for the polymer solution and seven injectors for the bore fluid. The experimental results show that the membrane microstructure, outer surface contour and shapes of lumen holes strongly depend on the spinning conditions such as dope formulation, bore fluid composition, air-gap distance and external coagulants. The evolution of membrane morphology as a function of spinning conditions has been elucidated by light transmittance. By carefully tuning the spinning parameters, polyacrylonitrile (PAN) rectangular membranes with a pure water permeation flux of 238 L/m(2) h bar for ultrafiltration have been manufactured. This performance is comparable to some literature works when only simple binary dope formulation is considered and no modifications are applied to the membrane. Compared to hollow fiber and flat sheet membranes, rectangular membranes do show a combination of the characteristics in terms of mechanical strengths. (C) 2011 Elsevier B.V. All rights reserved.