Asymmetric gas separation membranes were prepared by the phase inversion technique under different gelation conditions from polysulfone/N,N-dimethylacetamide (DMAc) solutions. The dual bath method was employed to control the skin layer properties : the cast film was immersed in 2-propanol bath and water bath in sequence. The membranes were characterized by the permeance of oxygen and nitrogen gases and the observation with scanning electron microscopy (SEM). A thin layer of silicone rubber (PDMS) was laminated on the surface of each asymmetric polysulfone membrane to eliminate the effect of defects in the skin layer. The oxygen permeance was inversely proportional to the square root of immersion time in the first (2-propanol) bath. The skin layer thickness determined by SEM observation increased with an increase in the immersion time in the first bath. For a given immersion time, the oxygen permeance decreased with an increase in the polymer concentration in the casting solution. Selectivity of oxygen over nitrogen also depended both on the immersion time in the first bath and the polymer concentration.