A series of uniaxial tensile loading-unloading tests is performed on isotactic polypropylene at room temperature. Prior to mechanical testing, injection-molded specimens are annealed for 24 h at temperatures T = 145, 150, 155, 158, 160, 163, and 165degreesC, which cover the entire region of high-temperature annealing temperatures. A constitutive model is developed for the elastoplastic behavior of a semicrystalline polymer at small strains. The stress-strain relations are determined by six adjustable parameters that are found by matching observations in cyclic tests. Fair agreement is demonstrated between the experimental data and the results of numerical simulation. It is shown that all material constants are affected by the annealing temperature, which is explained by changes in the crystalline morphology driven by thermal treatment. Some of the adjustable parameters experience finite jumps in the vicinity of the critical temperature T-c = 159degreesC. These jumps are attributed to the alpha(2) --> alpha'(2) phase transformation. (C) 2003 Wiley Periodicals, Inc.