An integrally skinned asymmetric polysulfone membrane was prepared by the phase inversion method. Its skin was defect-free so that an additional coating process was not necessary to caulk the skin defects for gas separation. Its separation factor of oxygen over nitrogen reached 5. Such a high separation factor could arise from the gelation of the interfacial layer between the dope solution and the gelation medium due to its increased polymer concentration before the liquid-liquid demixing. Experimentally, the polymer concentration at interface was controlled by the flux ratio of solvent to nonsolvent, R, which was, in turn, menipulated by introducing a cosolvent, ethyl acetate, Bacause the cosolvent has lower surface tension than the main solvent, N-methyl pyrrolidinone, the cosolvent-rich layer might be formed at the interface, which seemed to primarily control the flux ratio of solvent to nonsolvent. It was verified that an integrally skinned membrane without skin defects could be formed when the polymer concentration at interface was high before the onset of liquid-liquid demixing, and that the flux ratio played a crucial role in determining the membrane morphology.