Polymer/clay based composite films were prepared by incorporating sodium montmorillonite (Na+-MMT) and calcium montmorillonite (Ca2+-MMT) clay into poly(ethylene-co-acrylic acid) (PEAA). The dispersion of the MMT particles in the PEAA/MMT nanocomposites was examined by transmission electron microscopy (TEM) and X-ray diffraction (XRD). TEM indicated that the mean lateral dimensions of MMT particles in the PEAA/MMT nanocomposites was approximately 300 nm and their thickness was < 20 nm. The PEAA chains were intercalated into the interlayers of MMT. The NH4 + ions on the PEAA chains were replaced by Na+ or Ca2+ ions in the interlayer of MMT. The mechanical behavior of the PEAA/MMT nanocomposites with different MMT contents was examined using an Instron. The tensile strength increased with increasing MMT content. Since PEAA forms a network by Ca2+ ions, the tensile stress of the PEAA/Ca2+-MMT nanocomposites is larger than that of the PEAA/Na+-MMT nanocomposites with the same MMT content. The thermal properties of the PEAA/MMT nanocomposites were also discussed with reference to the TGA and DTA curves. TGA and DTA confirmed that the presence of Ca2+ ions allowed the PEAA to form a network.