Filling-type hollow-fiber membranes for separating organic-liquid mixtures were prepared by plasma-graft polymerization technique using porous hallow-fiber substrates. Methyl acrylate (MA) was used as the grafting monomer. From analyses with scanning electron microscope (SEM), optical microscope, XPS imaging and micro FT-IR, it was confirmed that grafted polymer filled the pores of the hollow-fiber substrates and the thickness of this grafted layer was about 20 mu m from the outer surface. Pervaporation (PV) performance of the grafted hollow-fiber membranes was measured and compared with the performance of the grafted flat-sheet membranes, The hollow-fiber membrane showed a lower separation factor and higher permeation rate than the flat-sheet membrane. This indicated that the mechanical stress exerted by the hollow-fiber substrate to suppress membrane swelling is lower than that of the flat-sheet substrate.