The hollow fiber membrane with a multi bore configuration has attracted much attention in recent years due to ease of potting during module fabrication and improved mechanical stability. The conventional multi-bore hollow fiber consists of seven or three bore channels and a round-shape outer geometry. In this work, novel tri-bore hollow fiber membranes made of Matrimid (R) and polyethersulfone (PES) materials with regular triangle-shape outer geometry have been fabricated. The fabricated membranes are advantageous in terms of uniform mechanical strength and enhanced permeation properties due to the evenly distributed membrane wall thickness. Experimental results suggest that the combined effects of rapid phase inversion, die swell, outer surface shrinkage and stress balance during the membrane formation are responsible for the unique membrane geometry. The micro-morphology, mechanical strength, pore size distribution and ultra-filtration rejection of the fabricated membranes were examined. We believe this work may provide useful insights for the fabrication of triangle-shape tri-bore membranes with other materials for various applications. (C) 2013 Elsevier B.V. All rights reserved.