Functionalized poly(ethylene terephthalate) (PET) fibers were synthesized with graft copolymerization of crotonic acid - methacrylamide (CrA-MAAm) comonomers by using benzoylperoxide as initiator onto PET fibers in an aqueous medium. The functionalized fibers were characterized by Fourier transform infrared spectroscopy, thermogravimetric analysis, differential scanning calorimeter, scanning electron microscopy, and X-ray diffractometry. The effects of reaction conditions, such as monomer mixture ratio, monomer mixture and initiator concentration, polymerization time, and temperature on grafting were investigated. In alone grafting of CrA, a graft yield as low as 2.2% was observed. However, the use of MAAm as a comonomer increased the amount of CrA inserted to the PET fiber up to 23.9%. An increase in the temperature between 65 °C and 95 °C and also, increase in monomer mixture concentration between 0.25 M and 1.00 M increased the grafting rate and saturation graft yield. The graft yield has shown an increase up to an initiator concentration of 1.0 × 10-2 M and decreased afterwards. The rate of grafting was found to be proportional of the 0.89 and 0.47 powers of CrA/MAAm and Bz2O2 concentrations, respectively. The grafting increased the crystallinity, dyeability with acidic and basic dyes, and water absorption capacity but decreased the thermal stability of the fibers. © 2008 Elsevier Ltd. All rights reserved.