Polyethylene terephthalate-multiwall nanotube (PET-MWNT) composites were prepared by the melt mixing technique of neat PET with a PET/MWNT masterbatch at various ratios. Uniaxially stretched samples were studied by means of Fourier transform infrared (FT-IR) spectroscopy. The populations of the trans conformers as well as of the trans non-crystalline conformers (TX structure) of the stretched samples were monitored with respect to the nanotube content. The effect that the presence of the nanotubes bears on the conformations of polyethylene terephthalate's chains under the application of uniaxial stress was evaluated. It was shown that the addition of the nanotubes leads to higher populations of trans segments in comparison to neat PET after the drawing process. In particular, the addition of small amounts of nanotubes (0–1.5%) leads to slight increase of the trans crystalline segments (TC structure) and a respective decrease of the gauche segments (G structure). The sample with 2% of nanotubes (NT) demonstrated a decrease of G structure in favor of the TX structure. Finally, only the sample with the highest NT content (3%) demonstrated severe increment of the TC. Thus, it was shown that when it comes to mechanically stretched PET low NT concentrations assist slightly the crystallization of PET. At higher NT contents PET adopts initially TX structure that eventually transforms to the corresponding crystalline TC structure as NT content increases.