Polymer/clay nanocomposites were prepared by melt intercalation using torque rheometer. Poly (ethylene-vinyl acetate) (EVA 18) with 18% wt. vinyl acetate was used as polymer, unmodified clay (Na+ montmorillonite, MMT), organo-modified clays I44 P, I31 PS and clays modified by in-situ radiation polymerization of charged monomer were used as the nanofiller. Unmodified clay was interacted with a charged monomer [2-(Methacryloloxy) ethyl]-trimethyl ammonium chloride in presence of water and irradiated to 10 kGy, 20 kGy, 30 kGy to load the clay with polymer. The maximum opening of 2.29 nm between the silicate layers of clay was obtained for 30 kGy irradiated clay as compared to 1.27 nm of untreated clay measured by X-ray diffraction (XRD) analysis. Structures of nanocomposites were characterized by ATR-FTIR, TEM (Transmission Electron Microscopy), XRD (X-Ray Diffraction) and PALS (Positron Annihilation Lifetime Spectroscopy). Close to 50% improvement in E-modulus and Tensile strength values was reached for nanocomposite prepared with 30 kGy irradiated clay, much better that the other nanocomposites except with organomodified clay I31PS all compared in equal loading. The improvement in mechanical as well as thermal properties was explained by structural analysis via XRD, TEM and PALS measurements.