AbstractCyclic deformation and stress relaxation after cyclic preloading in polypropylene were investigated by using an electrohydraulic, servocontrolled testing machine. The cyclic deformation tests were performed under various sets of strain rate, number of cycles, and strain amplitude in the as‐received sample and a quenched sample. The stress relaxation tests were made after a cyclic preloading in both samples. The distinctive shape of the hysteresis loop, termed a propeller‐like shape, is characteristic of the as‐received polypropylene with large spherulites, in marked contrast to the behavior of metals. The curves at strain amplitudes from ± 1.5% to ± 5% indicate a propeller‐like shape; these loops change into the steady state response as the number of cycles (N) is increased up toN= 30 to 50. The drop of stress in relaxation tests for the quenched samples is smaller than that for the as‐received samples at the same strain levels. This stress drop behavior reflects the difference of spherulite structure. The stress relaxation behavior depends on the morphology, the predetermined strain amplitude, and the process o