Polysulfone (PSf) membranes were prepared by the immersion precipitation method from casting solution of PSf and a water-soluble polymer, poly(vinyl pyrrolidone) (PVP), in dimethylacetamide. The effect of the molecular weight of PVP on the phase separation rate during the porous membrane formation was investigated by the light scattering method. The plots of scattered intensity against scattered angle showed maxima, which indicated that the phase separation occurred by spinodal decomposition. The structure growth rate remarkably enhanced by the addition of PVP with the lower molecular weight due to both the deeper quench depth and the lower polymer concentration brought about by the PVP leaching into nonsolvent bath. The increase of PVP molecular weight resulted in a decrease of the structure growth rate. Moreover, with the increase of the molecular weight of PVP, the membrane structures were changed from a macrovoid structure to a sponge structure.