The change in some optical properties with the creep extension is measured for polypropylene fibers at several constant applied loads. Multiple-beam Fizeau fringes in transmission are used to determine the mean refractive indices and the mean birefringence of these fibers at different values of creep extensions. Also, the effect of creep extension on the birefringence profile of the fiber is studied. The creep-extension dependence of these optical properties for polypropylene fibers is demonstrated using a stress-strain device connected to a wedge interferometer where the fiber is subjected to a constant load. The results show that under constant load the fiber will extend with time at a rate which decreases as time increases. An empirical formula is suggested to represent the variation of creep extension (%) of polypropylene fibers with time in the time interval of 0-55 min and the constants of this formula are determined. Microinterferograms are given for illustration.