A two-beam interferometric method is used to study the change of optical orientation functions and the molecular structure of annealed Egyptian poly( ethylene terephthalate) (PET) fibers. The acoustic method was used for measuring the density. The density results were used to calculate the degree of crystallinity of PET. It was found that annealing causes alignment to the fiber chains in the directions of the fiber axis. This alignment gives an increase in the optical orientation function and decrease in orientation angle. The value of Delta alpha/3 alpha(0), which depends upon the molecular structure of the polymer, remains constant. The obtained results of the optical and the density clarify that new reorientations occurred due to annealing at different conditions. The changes of the refractive index profile of annealed PET fibers are given. Microinterferograms and curves are given for illustration.