Microinterferometry (MIF), wide-angle X-ray scattering (WAXS), differential scanning calorimetry (DSC), and an Instron tensile tester (ITT) were used to determine the correlation between optical and structural properties of polypropylene (PP) fibers. For the purpose of the study a set of as-spun isotactic PP fibers were extruded by melt spinning at different draw-down ratios (DDR). The birefringence (Delta n), degrees of orientation, degree of crystallinity (chi), Young's modulus (E-e), and tenacity (tau) were determined for PP fibers at the different DDR. An equiangular orientation of PP fibers at particular DDR was predicted experimentally, and the transverse modulus (E-t) was estimated for the tested fibers. Empirical formulae were developed for correlating the fiber birefringence with some of the studied structural properties of PP fibers. POLYM. ENG. SCI., 50:200-208, 2010. (C) 2009 Society of Plastics Engineers