This study throws light on the change of the optical properties and some structural properties due to graft copolymerization of polydiallyldimethyl ammonium chloride (PDADMAC) and polyacrylamide (PAA) of nylon-6 fibers. Multiple-beam interferometric technique in transmission was used to study the change of the diameter, refractive indices, and birefringence of nylon-6 fibers at different graft yields. The results were utilized to investigate the isotropic refractive index, the mean polarizabilities per unit volume, dielectric constant, dielectric susceptibility, and surface reflectivity for nylon-6 and grafted nylon-6 fiber. The effect of grafted PAA onto modified nylon-6 fibers containing PDADMAC on the crystallinity was studied by X-ray diffraction. These results reflect good effect of grafting on the optical and structural properties of nylon-6 fibers. The opto-thermal properties of grafted PAA with different graft yields have been studied. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 117: 3255-3261, 2010