Diffusion studies of polyurethane-based membranes for hemodialysis have been performed. N-vinyl pyrrolidone which is hydrophilic and biocompatible was graft copolymerized on polyurethane. The membranes were prepared by solvent casting with varying the casting conditions, such as solvent evaporating time and temperature. Clouding phenomenon was not observed in the graft copolymer membranes during solvent evaporating period. The morphology of the membranes were investigated by scanning electron microscopy and pores were not found in any of casting conditions. The permeation characteristics of water and solutes which are the uremigenic agents were investigated on the graft copolymer membranes at 38℃. The membranes obtained from graft copolymer had good mechanical properties in wet state and comparable permeabilities of solutes in aqueous solution to those of Cuprophane.