A chloromethylated reaction was performed on the surfaces of polysulfone ultrafiltration hollow fibers and their derivative membranes, i.e., triethylaminated, ethylenediaminated, and acethylated fibers, were prepared. The modified fibers have a 30–60% insoluble region in chloroform, whereas nonmodified fibers can be dissolved in the chloroform. It is suggested that the insoluble parts of the fibers are highly cross‐linked due to the high degree of chloromethylation. The modified fibers showed a 50–98% rejection of polyethylene glycol 6000 at a feed concentration of 0.5 wt %, except for the triethylaminated fibers, which gave a negative rejection from −91 to −96%. It was found that the modified segments significantly influenced the rejection behavior of the solutes. Absorption of bovine serum albumin on the ethylenediaminated fibers at pH 7.1 was estimated from permeation measurements to be less than that of the other modified and nonmodified fibers. This effect is attributed to the hydrophilic surface of the ethylenediaminated fibers.