An automated two-beam interference Pluta microscope was used to measure the optical anisotropy or birefringence of polymer fibers. A sample of polypropylene (PP) fibers was used and cold-drawn to low draw ratios. The neck shoulder "necking" was micro-detected at draw ratio 1.3 and characterized using the measured thickness and birefringence along the drawn PP fiber. Birefringence profiles at different regions along deformed PP fiber were determined to confirm the effect of necking on these fibers. The influence of this phenomenon on the molecular orientation function of PP fibers was presented. The strong dependence of the birefringence on the thickness was formulated analytically at the necked zone of the drawn PP fiber.