The decay behavior of radicals produced on poly(viny1 alcohol) (PVA) fiber by photo-irradiation a t room temperature and the graft copolymerization of methyl methacrylate (MMA) on the irradiated PVA fiber were investigated. Two kinds of stable radicals showing singlet and triplet spectra were indicated for both unsensitized and ferric ion-sensitized samples, especially with the emphasis of triplet component radical. The decay of radicals was promoted by contact with various organic solvent-water solutions, which effects were in the order of dimethyl sulfoxide (DMS0)-water > acetone-water > water > dioxane-water > methanol-water. On the other hand, graft copolymerization of MMA on the preirradiated sample was effectively initiated with the aid of a little water or a mixed solution of organic solvent and water. Methanol and dioxane, which decay radicals milder than acetone and DMSO do, contributed to give a higher per cent grafting. As no initiation took place with the unirradiated sample, it is concluded that the ability of preirradiated samples to initiate graft copolymerization should be caused by the PVA fiber radicals, which are smoothly produced by photo-irradiation a t room temperature and show a triplet spectrum.