resently, extensive use of polymeric membranes both in industrial applications and research is widely acknowledged. In comparison to flat membranes, hollow fiber polymeric membranes due to their higher surface to volume ratio, have attracted more attention. In certain applications morphological properties of the membranes are of vital importance. Thus, production of hollow fibers membranes with defined morphology predominantly is conditional on prior identification of the parameters affecting the morphological properties of the hollow fiber membranes. In this work, the condition was achieved by simultaneous investigation of phase behavior of three ternary systems composed of non-solvent (water)/ solvent (DMF)/ glass polymers (PS, PEI, PSf, PES, PMMA) and rheological properties of the polymer-rich phases. Strictly speaking, in this work for the first time, superimposing rheological images on the ternary phase diagram was considered as a powerful and promising tool to gain more insight into the pore formation within hollow fiber membranes. The significant impact of the viscoelastic properties of the polymer-rich phases and the size of structure formation region (SFR) on the morphology of hollow fibers was explained by comparing the morphology of the membranes prepared from the solution with the same phase behavior and also the same zero shear viscosity (?0). The interplay of phase behavior and viscoelastic properties of the polymer-rich phases in determining the morphology of nano-structured hollow fiber membranes was also investigated. In this regard, it must be noted that in systems with instantaneous liquid-liquid phase separation, the mechanism of phase separation NG-L (nucleation and growth of polymer- poor phase) is dominant. In such systems, access to nanoscale pores (less than 50 nm) in membrane structure, is solely attainable by controlling the concentration of the initial solution as well as viscoelastic properties of the polymer-rich phases. This is achieved up on occurrence of phase separation through which the growth of the nascent nuclei is prevented due to the creation of polymer-rich phases with high viscoelastic modulus (G', G''). In such systems, the presence of polymer-rich phases in the upper layer slows down the penetration of non-solvent into the sub-layers. This is followed by delayed phase separation based on SD (spinodal decomposition) mechanism in sub-layers. Hollow fiber membranes with an interconnected network of pores at the nanoscale and no macrovoids can be produced by selecting a three-component system with delayed phase separation based on SD mechanism and polymer-rich phases with high viscoelastic modulus. It was found that both the number and size of macrovoids in hollow fiber membrane structure, not only is dependent on the ?0 of spinning solution and liquid-liquid phase separation time, but also is a function of so far ignored viscoelastic properties of the polymer-rich phases. Keywords: hollow fiber membrane, nano structure, ternary phase diagrams, viscoelastic properties of the polymer-rich phases, macrovoid.