A two-beam polarizing interference microscope with a microstrain device was used for measuring some optical and mechanical parameters for polypropylene (PP) fibers at room temperature (28 +/- 1degreesC). The changes in the molecular orientation were evaluated to obtain orientation factors f(2)(theta), f(4)(theta), f(6)(theta), , , and crystalline and amorphous orientation functions F-c and F-a' respectively. The shrinkage factor, uniaxial tension, true stress, molar refractivity R, surface reflectivity R', the crosslink density N-s, the chain entanglement density N-c, the segment anisotropy gamma(s), and the number of chains N' were calculated. In addition, the shrinkage stress was found to increase with the increase of draw ratio. The dielectric constant epsilon, the dielectric susceptibility eta, the average work per chain omega', and the constants of the stress-birefringence equation were obtained. Comparison between Hermans's optical orientation functions and the corrected formulas by de Vries are given. The values of fully oriented refractive indices n(1) and n(2) were found. The generalized Lorentz-Lorenz equation given by de Vries was used to determine the structural parameters of PP fibers. An empirical formula was suggested to correlate the changes in the evaluated parameters with different draw ratios, and its constants were determined. The study demonstrated changes on the molecular orientation factors and evaluated microstructural parameters as a result of an applied cold-drawing process. Relationships between the calculated parameters and the draw ratios, together with microinterferograms were presented for illustration. (C) 2003 Wiley Periodicals, Inc.