Refractive indices and birefringence for skin and core changes with annealing, produced by different annealing conditions in nylon-6 fibers, were measured interferometrically. Applications were carried out using multiple-beam Fizeau fringes in transmission to determine Cauchy’s constants and dispersive coefficients for the fibers. The resulting data were used to calculate the polarizability per unit volume for each layer. By optical microscopy, the geometrical parameters of the fiber cross section were found. The effect of temperature and time of annealing on the refractive index and birefringence for each layer were also investigated. The relationships between temperature and time of annealing with the optical parameters were drawn. Illustrations are given using microinterferograms and curves.