An asymmetric polyimide hollow fiber with a completely defect-free thin surface skin layer has been fabricated by using a dry/wet phase inversion process. Effects of the composition of spinning solution and the air gap height on the formation of the skin layer were investigated in detail. The hollow fiber showed a microporous structure at the inner surface and a defect-free skin layer at the outer surface. The outer diameter (OD) of the fiber was 500 mu m, with a wall thickness of 45 mu m and the calculated apparent skin layer thickness was 470 nm. Permeance and selectivity of O-2 and N-2 for the hollow fibers at 35 degrees C and 76 cm Hg have been measured. The (Q(O2)/Q(N2)) selectivity of the hollow fiber was 5.4 and exhibited a larger value than that measured in the dense and asymmetric polyimide flat membranes.