A series of uniaxial tensile loading-unloading tests are performed on injection-molded isotactic polypropylene at room temperature. In each test, a specimen is stretched up to the maximal strain is an element of(max) = 0.12 with a constant strain rate,, and retracted down to the zero stress with the same strain rate. The cross-head speeds vary from 5 to 200 mm/min, which covers practically the entire range of strain rates used in conventional quasi-static tests. A constitutive model is developed for the viscoplastic response of a semicrystalline polymer at small strains. The stress-strain relations are determined by five adjustable parameters that are found by matching the observations. Fair agreement is demonstrated between the experimental data and the results of numerical simulation.