Photo-induced graft polymerization of acrylic acid (AA) and methyl acrylate (MA) in the liquid and vapor phase, respectively, onto N,N-dimethyformamide (DMF)-pretreated poly(ethylene terephthalate) (PET) fibers in fabric substrate was studied. The effect of various synthesis conditions and DMF pretreatment on the graft yields on PET was investigated. The internal morphology and properties of DMF-pretreated and grafted PET fibers in the fabric were characterized using density and birefringence measurements, differential thermal analysis (DTA), Fourier transform infrared spectroscopy (FTIR), dyeing methods, and critical dissolution times. The grafting was promoted by increasing DMF pretreatment temperature and the amount of DMF retention in the PET. Increasing biacetyl and monomer flow time and irradiation time enhanced grafting. DMF pretreatment resulted in increases in total void content, degree of crystallinity, trans-isomer content, chain folding, segmental mobility, and molecular packing of the PET, but caused decreases in its amorphous orientation, intermolecular forces, and individual void size through longitudinal shrinkage, lateral swelling, and removal of oligomers. Subsequent graft copolymerization led to further changes in the internal morphology and properties of the PET. PET grafted with AA had a higher cohesive energy density, lower degree of molecular packing, and larger individual void size, but less total void content, lower segmental mobility, less chain orientation, and a lower degree of crystallinity. PET grafted with MA showed increases in total void content, individual void size, segmental mobility, and molecular packing, but showed decreases in chain orientation and degree of crystallinity.