Polyamide 6 (PA6)/clay nanocomposites were prepared using a single screw extruder with an ultrasonic die attachment. The die pressure and ultrasonic power consumption were measured at various clay concentrations. The structure and morphology of nanocomposites studied by X-ray diffraction (XRD) and transmission electron microscopy (TEM) indicated that the clay (Cloisite (R) 30B) was exfoliated within PA6 matrix at all clay concentrations, while clays (Cloisite (R) 15A and 93A) only intercalated. The complex viscosities of the nanocomposites were increased with clay loading and after ultrasonic treatment at lower amplitudes. The Young's modulus, stress and elongation at break, toughness and yield stress and strain of ultrasonically treated nanocomposites were significantly affected by the level of intensity of ultrasound. In addition to XRD, differential scanning calorimetry (DSC) and N-15 NMR techniques were used to investigate the structural changes in PA6/clay nanocomposites. The results indicated that the intercalated organoclay nanocomposites can induce generation of the gamma-form crystal of PA6 and substantially affect the arrangement of molecules in the alpha-form crystal. It was found that the addition of clay in the PA6 matrix leads to a decrease of crystallinity. However, ultrasonic treatment of nanocomposites increased crystallinity. A substantial reduction in oxygen permeability was observed by increasing concentration of clay. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 114: 2378-2387,2009