The water permeability coefficient D//1C//1 has been evaluated from the diffusion equation to understand the water transport characteristic in the cellulose acetate membranes prepared from the different concentrations and different acetyl contents. The active layer thickness of the asymmetric membranes was calculated by substituting the value of D//1C//1 in the diffusion equation. The solvent evaporation time has a pronounced effect on the formation of active layer thickness.