In this study poly(ether sulphone) (PES) asymmetric membranes and Polyamide (PA) composite membranes were used for gas separation at low pressures. The effect of PES concentration, contact time of sebacoyldichloride, addition of 1,6-hexaethylene diamine solution and PA active layer on the structures of PES support layer were studied. Changing the structure of PES membrane from channel-like to sponge form increased the selectivity in the gas separation process. However, this structural variation caused a considerable decline in gas permeability. The characteristics of top layer depend on the porosity and compactness of sub-layers structure. Active layer formation on the surface of support structure at PES concentrations of more than 25 %wt improved the oxygen selectivity considerably. The optimum conditions for preparation of PA active layer through interfacial polymerization process were elucidated in this paper. The PA composite membrane prepared at the PES concentration of 25 wt% and monomer concentration of 1 wt% provided the highest selectivity for oxygen enrichment process.