A hollow-fiber membrane extractor was prepared by plasma-graft polymerization. The plasma-graft polymerization technique made it possible to modify the surface of a porous hollow-fiber membrane of poly( tetrafluoroethylene) with a polyacrylamide layer. The thickness of the layer can be controlled by the concentration of a monomer and the plasma treatment time. The surface morphology of the grafted layer was analyzed with scanning electron microscopy (SEM) and electron spectroscopy for chemical analysis (ESCA). The results of the surface analysis revealed that the grafted layer covered completely the surface of a membrane. It was confirmed that the grafted layer of polyacrylamide can adsorb rare earth metals. Separation of rare earth metals was conducted in a hollow-fiber membrane extractor modified by plasma-graft polymerization. When the surface-modified membrane was used, the separation factor of erbium to yttrium increased by the interaction between the grafted membrane and the metal ions. It was found that the hollow-fiber membrane modified by plasma-graft polymerization was very useful as a separator for rare earth metals.