A defect-free as-spun hollow fiber membrane with an ultra-thin dense-selective layer is the most desirable configuration in gas separation because it may potentially eliminate post-treatments such as silicone rubber costing, simplify membrane manufacture, and reduce production costs. However, the formation of defect-free as-spun hollow fiber membranes with an ultra-thin dense-selective layer is an extremely challenging task because of the complexity of phase inversion process during the hollow fiber fabrication and the trade-off between the formation of an ultra-thin dense-selective layer and the generation of defects. We have for the first time successfully produced defect-free as-spun Torlon (R) hollow fiber membranes with an ultra-thin dense layer of around 540 angstrom from only a one polymer/one solvent binary system at reasonable take-up speeds of 10-50 m/min. The best O-2/N-2 permselectivity achieved is much higher than the intrinsic value of Torlon (R) dense films. This is also a pioneering work systematically studying the effects of spinneret dimension and hollow fiber dimension on gas separation performance. Several interesting and important phenomena have been discovered and never been reported: (1) as the spinneret dimension increases, a higher elongation draw ratio is required to produce defect-free hollow fiber membranes; (2) the bigger the spinneret dimension, the higher the selectivity; (3) the bigger the spinneret dimension, the thinner the dense-selective layer. Mechanisms to explain the above observation have been elaborated. The keys to produce hollow fiber with enhanced permselectivity are to (1) remove die swell effects, (2) achieve finer monodisperse interstitial chain space at the dense-selective layer by an optimal draw ratio, and (3) control membrane formation by varying spinneret dimension. (C) 2007 Elsevier B.V. All rights reserved.