This study is focused on the electrical and mechano-optical behavior of PET/CNT nanocomposite films under uniaxial deformation followed by relaxation processes. Simultaneous measurement of electrical, optical and true stress and true strain properties are followed during deformation of PET/CNT films of different CNT content (from 0.04wt% to 2.8wt %). This study helps to assess development functional properties of the film as they undergo heating- stretching –holding and cooling stages of shaping processes typical of packaging film manufacturing. During preparation, CNTs were melt-mixed with PET by twin-screw extrusion and the composites were compression molded into thin plaques. These plaques were quenched in an ice/water mixture to suppress crystallization. The plaques were stabilized above Tg and deformed in a custom built uniaxial stretching machine that allowed the simultaneous acquisition of electrical conductivity, optical birefringence (for composites up to 1 wt.% CNT) and true stress and true strain properties during deformation of PET/CNT films. The PET morphology developed was followed by DSC and WAXD. Our studies indicate strong dependency of the stress-strain behavior and electrical-strain behavior of PET/CNT films on the CNT concentration. The electrical percolation is not achieved due to the low content of CNT used; nevertheless, for 2,8wt % the surface resistivity decreases considerably. During deformation, the surface resistivity increases exponentially with the increase of strain applied. During relaxation stage, the stresses relaxed and surface resistivity also declined during this process.