Nylon 6/clay nanocomposites of various clay minerals with different cation exchange capacity (CEC) were prepared by in situ polymerization method and properties like mechanical, thermal and gas permeability were compared with respect to pure nylon 6 (N6) polymer. The functional groups of clay and epsilon-caprolactam (CPL)/clay mixture are detected by using Fourier transform infrared (FTIR) spectroscopy. Wide angle X-ray diffraction (WAXD) and transmission electron microscopy (TEM) techniques were employed to study the morphology of the nanocomposite material. The intercalated or exfoliated morphology of the nanocomposites only depends on the type of clay, not on the CEC. The thermal property of Nylon 6/clay nanocomposites were examined by using differential scanning calorimetry (DSC) which indicates that the exfoliated nanocomposites exhibit better thermal properties. The CO2 gas barrier property measured by gas permeability analyzer (GPA) confirms that better dispersion of clay in the polymer matrices provides much higher gas barrier property.