By using co-extrusion and dry-jet wet-spinning phase-inversion techniques with the aid of heat-treatment at 75 degreesC, we have fabricated dual-layer polyethersulfone (PES) hollow fiber membranes with an ultrathin dense-selective layer of 407 Angstrom. To our best knowledge, this is the thinnest thickness that has ever been reported for dual-layer hollow fiber membranes. The dual-layer hollow fibers have an O-2 permeance of 10.8 GPU and O-2/N-2 selectivity of 6.0 at 25 degreesC. The effects of post heat-treatment on the membrane morphology and gas separation performance were also studied. It was found that heat-treatment at 75 degreesC improves the gas permeance and ideal selectivity, while heat-treatment at 150degreesC results in a significant reduction in both permeance and selectivity due to the enhanced substructure resistance. SEM pictures confirmed that a higher heat-treatment temperature can significantly reduce pore sizes and the amount of pores in the substructure immediately underneath the dense-selective layer. (C) 2004 Elsevier B.V. All rights reserved.