Thermodynamic and kinetic aspects of phase inversion process were investigated to elucidate the mechanism of asymmetric membrane formation. A solubility parameter map of polysulfone was obtained. A solvent (NMP), a solvent additive (FA : formic acid) and a non-solvent(ethanol) were chosen on the basis of the solubility parameter map. A series of cloud points measurement resulted in a phase diagram illustrating that the polymer solution is thermodynamically unstable with increasing amount of FA in the solvent mixture. An asymmetric membrane prepared was characterized in terms of the skin structure and the cross-sectional morphology as well as the oxygen permeability. The diffusion coefficients of non-solvent into polymer solutions from the precipitation kinetics were also obtained. The sponge-like membrane was prepared with the system of the rapid exchange rate of non-solvent and solvent. This observation is contradictory to common observations. The macrovoids formation was also investigated by an optical microscope and seemed to be attributable to the non-solvent intrusion through the surface microcracks at the surface, which, in turn, related to the viscosity and surface tension of polymer solution.