The effect of heterogeneity of a dope polymer solution on membrane morphology has been investigated. Heterogeneity was developed in polyethersulfone solutions of either gamma-butyrolactone (gamma-BL) or a mixed solvent of gamma-BL/N-methyl-2-pyrrolidone, which exhibit gelation and/or phase separation with aging time. Two different processes for membrane preparation have been employed; Process I, heterogeneity-developed dope solution was cast on a glass plate with a knife for uniform thickness, and immediately immersed into a nonsolvent form of the membrane. Process LI, a dope solution was cast on a glass plate and then aged in a closed environment. After a certain aging time, it was immersed into a nonsolvent to be coagulated. The developed heterogeneity was destroyed markedly during casting in Process I, whereas the heterogeneity was maintained until the moment of immersion into a coagulation medium in Process II. Membranes prepared by Process I were finger-like regardless of the aging time, whereas Process II yielded macrovoid-free and sponge-like membranes having enhanced gas permeance and mechanical strength at long aging times. From these results, it was concluded that the morphology and transport properties of a membrane were able to be controlled by the solution structure, in particular, the solution heterogeneity in the cast film.