The chemistry of free-radical graft copolymerization initiated with peroxomonosulfate (PMS)-thioglycolic acid (TGA) redox system has been investigated by using 4-vinyl pyridine (4VP) as a model for nylon 6 fiber in aqueous solution under nitrogen atmosphere. Effects of concentration of 4VP, PMS, TGA, nylon 6, time, and temperature on R-h and graft parameters were studied. The FTIR spectrum of nylon 6-g-4VP was reported. Water retention capacity (WRC) of the grafted fiber was tested.