Polyethylene terephthalate (PET) nanocomposite films (with 3 wt.% Cloisite 30B) were prepared by cast extrusion followed by uniaxial stretching, using chill rolls. Two screw profiles with different mixing elements under different screw speeds (N) and feeding rates (Q) were used to prepare PET/clay nanocomposite (PCN) films. Transmission electron microscopy (TEM) and wide angle X-ray diffraction (WAXD) showed that the clay layers were aligned in the machine direction (MD). XRD patterns depicted that the interlayer distance of clay platelets in the state of intercalation is somehow independent of the processing conditions, but the macro-scale characterization, including barrier and mechanical properties, showed that the level of clay layer delamination was affected by processing conditions. The results reveal that the applied strain has stronger effect than residence time on the barrier and mechanical properties. At the highest screw speed (N = 250 min −1), 27% reduction in oxygen permeability and 30% improvement in tensile modulus were obtained for the more severe screw profile.