This article sheds light on the effect of creep strain [epsilon(t); %] on the optomechanical properties and some structure properties of terylene fibers at several constant applied loads. Automated multiple-beam Fizeau fringes in transmission were used with a mechanical creep device attached to a wedge interferometer where the fiber was subjected to a constant load. This technique was used to determine the mean refractive indices and the mean birefringence values of terylene fibers under different conditions for epsilon(t). The obtained optical results were used to evaluate the optical orientation function, optical stress coefficient, density, crystallinity, and mean-square density fluctuation with epsilon(t). The obtained results show that, under a constant load, the terylene fibers extended with time, the rate of which decreased with time. An empirical formula is suggested to represent the variation of epsilon(t) of terylene fibers with time, and the constants of this formula were determined. A mechanical model is proposed to represent epsilon(t) of terylene fibers, which consists of two Kelvin elements combined in series, which were used to provide an accurate fit to the experimental creep curve. The stress-strain curve via creep was studied to determine some mechanical parameters of the investigated fibers: Young's modulus, yield stress, and yield strain. Illustrations with microinterferograms, graphs, and tables are given. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 113: 516-525, 2009