The effects of the molecular weight of polyvinylpyrrolidone (PVP) on the precipitation kinetics during the formation of an asymmetric polyacrylonitrile membrane and the resulting membrane morphology were investigated. The precipitation kinetics of a polyacrylonitrile membrane was explored using the light-transmission method and optical microscopic observation, with an emphasis on the effective diffusion coefficient of the nonsolvent as a measure of the solvent/nonsolvent exchange rate. The retarded precipitation behaviors were not observed in all dope solutions. Also, the phase-separation rate was steeply decreased with the amount of PVP. It was deduced that increase of the viscosity of the polymer solution upon the addition of PVP hindered the intrusion of the nonsolvent, resulting in a decrease of the phase-separation rate. When the same amount of PVP with a different molecular weight was used, the precipitation rate decreased with increase of the molecular weight of PVP, Therefore, the nonsolvent tolerance, the interaction between PVP and H2O, as well as the viscosity are regarded as important factors influencing the precipitation rate. The effective diffusion coefficient of the nonsolvent is closely related to the nonsolvent tolerance of the polymer solution, resulting from a pseudoternary phase diagram. The use of high molecular weight PVP strongly depressed the formation of macrovoids inside of the membrane and allowed a dense skin layer to form.