Carbon nanotubes are among the stiffest and strongest fibres known and they are thus considered as ideal fillers for polymeric fibre reinforcement. Carbon nanotube polymer composites have consequently attracted huge academic and industrial interest with thousands of relevant research works being published every year. In current work, we present a quite integrated study of multiwall carbon nanotube (MWCNT)-reinforced poly ethylene terephthalate (PET) composites prepared at laboratory scale along with industrial melt-spun fibres. For an optimum dispersion of the nanomaterial in the polymer matrix, we proceeded to appropriate functionalization of multiwall carbon nanotubes. The morphology of the composites was inspected by scanning electron microscope and transmission electron microscope, while the physical properties, such as crystallinity and orientation, by differential scanning calorimetry, X-ray diffraction and Raman spectroscopy. The addition of well-dispersed carbon nanotubes acts as a nucleation agent increasing the crystallization of poly ethylene terephthalate matrix, however, decreasing the orientation of either films or fibres. Carbon nanotubes /poly ethylene terephthalate polymer composite films present an increment of Young’s modulus and tensile strength to detriment of failure strain; namely, stiffness is accompanied by a less ductile behaviour. With the addition of carbon nanotubes to poly ethylene terephthalate fibres, a decrease in shrinkage and only a slight improvement in dimensional stability was attained; this once more explains the slow growth of their commercial applications since the mechanical properties of these materials still remain a fraction of the expected theoretical values.