The chemical grafting of acrylamide-methacrylic acid (AAM-MAA) mixture onto poly(ethylene terephthalate) (PET) fibers using azobisisobutyronitrile as a chemical initiator was investigated. The use of MAA as a comonomer increased the amount of AAM introduced to the PET fiber up to 33.0%, while the grafting of AAM onto fibers alone gave low graft yields. This synergistic effect was found to be at its highest when an AAM-MAA mixture having 30 wt % AAM was used. The grafting increased dyeability with both acidic and basic dyes, and increased diameter and decreased the density of the fibers. The thermogravimetric analysis results revealed that the decomposition temperature of the fibers decreased with grafting. The electron micrographs showed that grafting changed the surface morphology of the fiber and a shell-like heterogeneous structure occurred at the surface at high graft yields.