The changes in mechanical properties of PET cords annealed under various conditions and PET cord-rubber composites cured under some conditions were discussed by means of the results of thermal contractive force and discontinuous stress relaxation measurements. Thermal contractive force was measured at a heating rate of 20°C/min in the range of 25°C to temperatures below melting point of PET. Discontinuous stress relaxation was measured under initial load of 3 g/denier (194N). The duration for the release of stress is zero or 5 min. The composite consisting of cords treated at 200°C, which showed the generation of contractive force in a lower temperature range was higher thermal contractive force than that of cords treated at 240°C. The thermal contractive force of the composites was affected not only by the treated conditions of the cord but also by the cord tension on preparing the specimen. In the discontinuous stress relaxation test, the stress of the cords and the composites increased with repetition of stretching. When the strain recovery of the amorphous region of the cord hardly occurred in duration of intermission, the stress of the cord and composite increased extremely. It seems that the increase of stress under the discontinuous stress relaxation test is due to the strain hardening of tie molecular chains. The stress relaxation of the composites were larger than that of the cords. It seems that this phenomenon is influenced by the tie molecular chains of the cord which are relaxed rapidly at the initial stage of thermal shrinkage.