The use of polymeric membranes has been vastly increased within relatively short time period. This phenomenon is due to the fact that membrane constitutes the cornerstone of separation process. Various membranes such as flat sheet and hollow fiber membranes are available. In general, low production cost together with low material cost have led to preference of polymeric membranes to inorganic types in wide range of applications. In this regard, the use of hollow fiber membranes has gained paramount importance. Hollow fiber membranes can be used in various applications such as microfiltration, nanofiltration, forward osmosis, gas separation, carbon dioxide absorption, distillation, dialysis etc. Each application requires a membrane with defined morphology. Therefore, the ability to tailor morphology of the hollow fiber membrane is the vital criterion up on which the membrane can be designed. This work attempts to eliminate the ambiguities surrounding morphological aspect of the hollow fibers membrane and facilitate their design by employing liquid-liquid phase separation and crystallization processes. Hollow fiber membranes were produced using semi-crystalline polymer, i.e. poly (vinylidene fluoride) (PVDF). The production of the hollow fiber samples includes formation of crystals, gelation process and liquid-liquid phase separation. The first part of the work is concerned with investigating the effect of participating solvents (DMF, NMP, 2P), type of PDVF and concentration of the polymer solution on the formation of nanostructure (pores and crystalline phases) in the PVDF hollow fiber membranes. Various solvents by affecting phase behavior and gelation of the non-solvent/solvent/polymer ternary system, change viscoelastic properties of PVDF dope. These effects not only resulted in changes in the dimensions of macro-voids and cavities but also caused changes in the crystalline phases in the PVDF membranes structure. Additionally, the use of PVDF with different melt flow indexes (MFI), leads to changes in the viscoelastic properties of the PVDF solutions as the result of which membranes with different structures are produced. It was found that ternary phase diagram alone is an inadequate tool by which membrane structure can be predicted particularly membranes with nanostructure. Thus, in order to analyze and predict the structure of the hollow fiber membranes, simultaneous consideration of rheological properties and ternary phase behavior of different solutions must be used. Appropriate membranes for CO2 absorption in the form of gas-liquid membrane contactor with high CO2 absorption capability, were designed and manufactured. The membrane design was based on the interaction of phase behavior and properties of gelation polymer solutions. The second part of the work is concerned with investigating the effect of the relative position of the spinning solution in the ternary phase diagram with respect to binodal and gelation boundaries. Maintaining constant polymer volume fraction ( ) and varying volume fraction of the solvent ( ) and non-solvent ( ), spinning solutions were prepared in various locations in the phase diagram. The solutions were prepared using one type of PVDF and 2P solvent. It was observed that changing in relative position of spinning dope on the phase diagram with respect to binodal and gelation boundaries, results in changes in formation and growth of crystalline phases. These variations in turn affect the mechanical properties of the membranes such as strength and Young's modulus. Additionally, it was found that shifting of the dope from the polymer/solvent axis, results in formation of a dual structure in PVDF membrane. The formation of the dual structure leads to significant improvement of disperses dye rejection by the membranes. Keywords: Poly (vinylidene fluoride), Hollow fibers, Membrane, Phase behavior, Gelation, Nanostructure, Crystals, Liquid-liquid phase separation.