Asymmetric hollow fiber membranes were prepared from a cardo polyamide by using a wet phase inversion process. Polymer dope contained the cardo polyamide (20 wt. parts), LiCl (5 wt. parts) and NMP (100 wt. parts), and coagulant was water both for bore fluid and a coagulation bath. The membrane showed the selectivity of oxygen over nitrogen of 6.0, which was similar to that of a dense film, and the oxygen permeation rate of 12 x 10(-6) cm(3) (STP)/(cm(2) s cmHg) (12 GPU, and 9.0 x 10(-11) m(3)/(m(2) s Pa)) at 25 degreesC. The membrane was stable up to 240 degreesC with an O-2/N-2, selectivity of 2.3. Morphology of the membrane was investigated by scanning electron microscopy (SEM) and an ultrathin sectioning method with transmission electron microscopy (TEM). It was observed that the cardo polyamide hollow fiber membrane had the skin layer in the inside surface, whereas porosity on the outside surface. The inner skin layer was constructed from nodular substances, and classified three parts, that is, a topmost layer, an underlying transition region, and a porous substrate, because of the difference in morphology. The topmost layer consisted of rounded cylindrical nodules elongated along spinning flow, whose average dimensions were 6.5 +/- 1.5 nm in diameter and 15 +/- 4 nm in length. The orientation of the nodules along spinning flow was observed. The nodules were tiered in double to form the dense, defect-free topmost layer, of which the thickness was about 11 nm on the average. In the underlying transition region, even though the nodule diameter was similar, there observed many micro voids, and its thickness was about 80 nm. And, in the porous substrate, nodules became larger as receding from the surface. On the other hand, the outer surface of the hollow fiber membrane is porous. Gas permeation data estimated a defect-free skin thickness of about 100 nm, and indicated that the topmost layer and the underlying transition region might be a skin layer for as permeation, even though micro voids existed in the underlying transition region in TEM observation. (C) 2004 Elsevier B.V. All rights reserved.