Influence of temperature on creep deformation for polypropylene PP fiber under a constant load was studied interferometrically. The automated multiple-beam Fizeau system in transition was equipped with a mechanical creep device attached to a wedge interferometer. This system was used to determine the optical properties (n(II), n(I) and Delta n) of PP fiber during the creep process at constant loading with varying temperature. The creep compliance was drawn as a function of both time and temperature. An empirical formulat was suggested to describe the creep compliance curves for PP fibers and the constants of this formula was determined. Two Kelvin elements combined in series were used to provide an accurate fit to the experimental compliance curves. The stress-strain curve via creep was studied to determine some mechanical parameter of PP fibers, Young's modulus E, yield stress sigma(gamma), and yield strain epsilon(gamma). The optical orientation function f(theta), the dielectric constant d, the dielectric susceptibility chi, the surface reflectivity ($) over barR, and the average work per chain W" were also calculated. (C) 2008 Wiley Periodicals, Inc. J Appl Polym Sci 110:761-768 2008