Two modified hollow fiber membranes, the chelating hollow fiber membrane with iminodiacetic acid and the cation-exchange hollow fiber membrane with sulfonic acid group (-SO3H), were prepared by radiation-induced grafting of glycidyl methacrylate onto polyethylene hollow fiber membrane and its subsequent iminodiacetation and sulfonation. The adsorption characteristics of Co2+ and Cs1+ for the 2 hollow fiber membranes were examined when the solutions of Co2+ and Cs1+ permeate across the 2 membranes, respectively. Without regard to the chelating membrane with iminodiacetic acid group and the cation-exchange membrane with sulfonic acid group (-SO3H), 2 membranes were observed to adsorb Co2+ higher than Cs1+. The adsorption curves of Co2+ by IDA group-chelating fiber membrane in the presence of Na1+ and Ca2+ showed that the chelating hollow was found to have a very high selectivity for Co2+, even though there is a high concentration of Na1+ and Ca2+ in the inlet solution.