Two-beam interferometry and density methods previously measured are used to study the changes in optical properties of annealed and cold drawn polyester (PET) fibers. Some structural parameters in the present work are determined, such as form birefringence, the number of molecules per unit volume, the virtual refractive index, the harmonic mean polarizability of the dielectric, the harmonic mean specific refractivity, and the isotropic refractive index. Also, the distribution of segment at an angle with respect to the draw ratio and the electric polarizability constant (Delta alpha/3 alpha(0)) are determined. The generalized Lorentz-Lorenz equation given by de Vries is used to determine PET fiber structure parameters. Comparison between the results have been made with the Hermans optical orientation function.