Radical graft polymerization is an effective method for surface modification of inert and hydrophobic poly(ethylene terephthalate) (PET) fibers, but is lacking grafting efficiency and control. Several key interactive and controlling elements of the graft polymerization reaction were identified, and corresponding affinity characters of each reaction component were quantified by using Hansen solubility theory. Proper control of the radical graft polymerization reaction on PET fiber surfaces was achieved.