The membrane morphology was readily controlled by the gelation of dope solution before immersion into a coagulation medium. In particular, macrovoid-free, sponge-like integrally skinned asymmetric polyimide membranes with improved selectivity were obtained by the chemical gelation of cast solution film. Cross-linking was performed by ultraviolet irradiation for benzophenone containing polyimide dope solution. Cross-linking maintained the structure of cast solution him until the moment of immersion into a nonsolvent bath. Macrovoid decreases with irradiation time on the cast solution Blm. Sponge-like integrally skinned asymmetric membranes were obtained with sufficient irradiation. Gas permselectivity increased significantly with the progress of cross-linking on the cast solution him. Membrane morphology and subsequent transport properties can be controlled both by the extent of cross-linking and by the concentration of dope solution,