The changes produced by the effects of annealed and drawn fibers on the microstructure and macrostructure of nylon 66 fibers are considered. The optical properties and strain produced in nylon 66 fibers under different conditions are measured interferometrically at room temperature. Structural parameters are calculated such as the average work per chain, the work per unit volume, the reduction in entropy due to elongation, and the work stored in the body as strain energy. The evaluation of the density aided the calculation of the crystallinity, the mean square density fluctuation, the isotropic refractive index, the harmonic mean polarizability of the dielectric, and the harmonic mean specific refractivity. In addition, the resulting data are utilized to calculate the optical stress coefficient and the optical configuration and to apply the Mooney-Rivlin equation to determine its constants. Also, the number of crystals per unit volume and the average orientation angle for uniaxial stretching are calculated by the extension ratio. The relations between the optical, mechanical, and thermal changes with different parameters are given for the studied fibers.