Controlled reactions on the inner side, outer side, and both sides of the surfaces of polysulfone ultrafiltration hollow fibers with propane sultone and Friedel‐Crafts catalysts were developed. EPMA measurements and MTR spectra for the chemically modified fibers suggested existence of CH2CH2CH2SO3− segments on the modified surfaces. The modified fibers were found to have smaller molecular weight cut‐off than nonmodified fibers, and the fibers modified on the internal surfaces gave better rejection of polyethylene glycol 6000 than those modified on the external surfaces, although the fibers that reacted with solution of the propane sultone and SnCl4 at 70°C and 80°C showed negative rejection of the polyethylene gylcol. Absorption of polyethylene glycol on the modified fibers is estimated to be less than the nonmodified fibers from the flux ratios of aqueous polyethylene glycol solution to pure water. This effect is attributed to the heparinlike active group of modified segments.