Considering the rapid progress in applications of membrane separation technologies in various industries and the importance of asymmetric membranes morphology in governing the separation performance in membrane systems, control of this parameter in the membrane fabrication process is regarded as a prominent subject in the field. Hence, investigation on the rheological properties of polymer solutions, including viscoelasticity and gelation behavior and their influence on the membrane formation and morphological structure of membranes including flat sheet and hollow fiber membranes is a prerequisite to produce asymmetric membranes with desirable characteristics. Phase inversion is the most widely used technique for the preparation of asymmetric membranes and the two major mechanisms of liquid-liquid and solid-liquid demixing in membrane fabrication process affect the morphology of the membranes. Therefore, controlling the phase inversion in the early stage can greatly influence membrane microstructure. The rheological behavior of polymer solutions during the fabrication of membranes as well as other factors that influence the morphological structure of the membranes are evaluated in the present study. In addition, the principles governing the rheology of polymer solutions such as viscoelasticity, shear and extensional viscosity play a crucial role in determining the membrane morphology and separation performance. Due to the interaction of the rheology of polymer solutions and phase inversion, the effects of changes in the rheological properties on the phase inversion and the formation of membranes with different structures and morphologies are studied. Furthermore, in addition to the analysis of the effect of the relaxation time and gelation mechanisms, discussions are provided about the determination of the final membrane morphology considering the competition between the domains growth and gelation rates.