An effective method for the grafting of poly(acrylic acid) on polypropylene fibers has been developed, using diphenyl and a dispersing agent (NNO) in the grafting bath as additives to facilitate the grafting process. The method makes it possible to obtain high grafting degrees of poly(acrylic acid) on polypropylene fibers with a minimal quantity of homopolymer as a side product. The effect of grafting degree on the moisture absorption and swelling of the modified fibers has been examined and described with mathematical equations.