The polymer chains containing an amidoxime (AO) group capable of specifically collecting uranium in seawater were grafted onto a microporous hollow-fiber membrane. Crosslinking of the grafted polymer branches by bivalent ions such as Mg and Ca ions in seawater permitted permeation of seawater through the AO-group-immobilized micropores. The logarithm of the ratio of uranium concentration at the outlet to that at the inlet linearly decreased with an increasing residence time of seawater across the AO hollow-fiber membrane, which indicates that the chelate formation of the AO group immobilized on the microporous membrane with uranyl species governed the overall uranium uptake rate.