A simple model has been developed which relates membrane surface skin layer thickness to solvent evaporation time. This model can be used to determine other important membrane morphology parameters such as pore number (or pore density) in every pore size distribution on the membrane surface. With pore number becoming available, membrane morphology can now be more completely, explicitly, and quantitatively characterized. The changes in membrane morphology parameters during the solvent evaporation step of making cellulose acetate butyrate (CAB) membrane (in acetone) were investigated based on the Surface Force-Pore Flow (SFPF) model and a new model developed in this work. The analyses show that in the solvent evaporation step of making CAB membranes, there exist three distinct morphology stages or phases associated with solvent evaporation time : (I) intrinsic pore-determining phase, (II) phase of forming dominantly massive small pores, and (III) phase of forming dominantly large pores. It is the combined effect of several morphology parameters that determines the membrane performance, which in this case yields a high solute separation and permeate rate when solvent evaporation time is set at 60 seconds.