Oxygen transmission rate (OTR), mechanical and thermal properties of uniaxially stretched aliphatic and aromatic nylon films and their in-situ polymerized nanocomposites with 4 wt% clay were studied. Stretching was carried out at 110 ºC to draw ratios from 1.5 to 5. The changes in crystalline and clay orientations were examined via Fourier Transform Infrared (FTIR) and wide angle X-ray diffraction (WAXD). Structural analysis conducted using X-ray diffraction (XRD), Differential Scanning Calorimetry (DSC) and Modulated DSC revealed significant differences in crystallinity and crystal type in the two types of nylons before and after hot drawing. Clay platelets showed a γ-nucleating effect for aliphatic nylon matrix. Stretching the aliphatic nylon films increased the modulus by about 100 % and yield stress by about 200 % whereas it reduced the elongation at break. Gas permeation measurements revealed about 33% reduction in oxygen permeability for stretched aliphatic nylon films at low draw ratios. Hot stretching did not affect modulus and oxygen permeability of the aromatic nylon film and its nanocomposite. Uniaxially stretching enhanced clay orientation in the plane of the films. In addition, planar orientation of γ-phase crystal in nylon films and their nanocomposites increased in both the machine and transverse directions after stretching. Clay particles inhibited γ-phase crystal orientation in the nanocomposites compared to neat nylon. Key words: Nylon, Nanocomposites, Permeability, Orientation, Uniaxial Stretching.