Poly(vinyl alcohol) membranes, having integrally skinned asymmetric structures, were prepared by using 2-propanol and cosolvent composed of N-methyl-2-pyrrolidone and water, by phase inversion processes. In this work, the effects of cosolvent on the formation of integrally skinned structure of the asymmetric poly(vinyl alcohol) membranes were investigated and discussed from the morphological point of view. Experimental result indicated that the resulting membranes exhibited closely overall morphology change from a uniform cell structure to an asymmetric structure with a dense skin layer and a nonporous structure, by increasing the N-methyl-2-pyrrolidone ratio in the cosolvent. For this phenomenon, it was considered that as the N-methyl-2-pyrrolidone ratio in polymer solutions increased, due to a poor miscibility in between them, the inflow rate of 2-propanol decreased gradually, while the overall liquid-liquid demixing pattern between a cosolvent and 2-propanol depended on the outflow rate of the cosolvent during phase inversion.