Ultrafiltration of proteins using hydrolyzed polyacrylonitrile hollow fiber was investigated in this work, Polyacrylonitrile hollow fiber was spun and hydrolyzed with NaOH aqueous solution under various conditions. A thin layer of polyacrylic acid was formed on the inner surface of the hollow fiber, and the ionic density on the hydrolyzed surface was determined through titration. The hydrolyzed hollow fiber was characterized with the hydraulic permeability and the retention of dextran. A sharp decrease in the hydraulic permeability was observed between pH 5 and 6, possibly due to the swelling of the hydrolyzed layer. The retention of dextran slightly increased with the ionic density of the hydrolyzed hollow fiber. The retentions of two proteins, myoglobin and cytochrome c, were measured over a range of pH values (4 similar to 10). The results show that the retention of protein changes with pH, and is the lowest at the isoelectric point. Separation of mixtures of cytochrome c and myoglobin was performed at pH 5, 7, and 9 using hydrolyzed PAN hollow fiber. The selectivity of the cytochrome c over myoglobin was about 40 at pH 5, about 1 at pH 7, and about 0.5 at pH 9.