In this work, changes in the structure of polyester [poly(ethylene-terephtalate) (PET)] fibers after annealing at constant temperature (120 and 140 degrees C) and different times were studied interferometrically. The density of annealed PET fibers were measured by a system based on vibrating string. The Pluta polarizing interference microscope was used to determine the optical parameters of these fibers. The density and optical results were used to calculate the degree of crystallinity, the form birefringence, the number of monomeric units per unit volume, harmonic mean polarizability of the dielectric, harmonic mean specific refractivity, and the virtual refractive index. The behavior of fiber crystallinity at different annealing conditions were discussed with different optical parameters. Hermans optical orientation function have been compared with the generalized Lorentz-Loranz equation given by de Vries. Microinterferograms and curves are given for illustration.