Hollow fiber membranes for hemodialysis were prepared from polyethersulfone (PES) and poly(vinyl pyrrolidone) (PVP) (PES/PVP=18/3 and 18/6 by weight) solution in N,N-dimethylacetamide by the dry wet spinning method. Hollow fibers were then heated either in hot water (95°C for 30min) or in air (150°C for 5min). These membranes were characterized by ultrafiltering dextrans of different molecular weights (200ppm in water) at room temperature and an operating pressure of 5psig. It was observed that the water flux of the hollow fiber increased significantly when heat-treated in water, while decreased when heat-treated in air. On the other hand, molecular weight cut-off (MWCO) of the hollow fiber increased slightly when heat-treated in water, while decreased drastically when heat-treated in air. The morphology of the surfaces of the hollow fibers was studied by atomic force microscope (AFM) in terms of surface roughness. The roughness of both inner and outer surface decreased upon heat-treatment, either heated in water or in air. The membrane heat-treated in air exhibited the lowest roughness parameter. SEM images also showed that the surface morphology of membranes was different before and after heat-treatment. Discussions are made on the basis of MWCO and AFM results. The performance data of the hollow fiber heated in air at 150°C was found to be the most appropriate for hemodialysis application. The hollow fiber membrane prepared from the blend ratio of PES/PVP=18/3 showed slightly higher flux than the hollow fiber membrane prepared from a solution with PES/PVP ratio of 18/6.