Polyethylene terephthalate (PET) exhibit good balance of mechanical strength, thermal properties and barrier properties that makes it valuable for commodity and engineering applications. To pursue to high performance applications where enhanced electrical properties of PET such as electrostatic dissipation or electromagnetic shielding are required, PET composites with carbon nanoparticles present great potential. Carbon nanotubes (CNT) have high modulus and strength, outstanding thermal and electrical conductivity, and thus are serious candidates for PET nanocomposites for these specialty applications. In the present study we report the preparation of PET/CNT nanocomposites with compositions ranging from 0.04 to 2.8 wt.% and the analysis of the influence of CNT on the deformation mechanism of PET. The CNT were melt-mixed with PET by twin-screw extrusion and the composites were compression molded into thin plaques. The plaques were quenched in an ice/water mixture to suppress crystallization. PET and nanocomposite plaques were stretched on a universal tensile testing equipment under controlled temperature and deformation rate. The plaques were equilibriated in the rubbery state above Tg, deformed, and immediately cooled by admitting CO2 into the temperature controlled chamber. The PET morphology developed during stretching was analyzed by birefringence (for composites up to 1 wt.% CNT), wide angle X-ray diffraction and differential scanning calorimetry. It was observed that the initial degree of crystallinity was below 10% for PET and nanocomposites, except for the nanocomposite with 2.8 wt.% of CNT, that presented 20% crystallinity. The mechanical response of the nanocomposite plaques was strongly dependent on the CNT composition. Two opposite trends were observed: at CNT content below 1 wt.% strain hardening was delayed; at higher CNT content the stress increased rapidly. The latter effect was mainly due to the CNT contribution.