polyolefin/clay nanocomposites: in situ polymerization, solution method and melt mixing. Melt mixing is the easiest and most cost effective method to produce nanocomposites. To melt-mix nonpolar polyolefin with polar clay platelets, clay surface must be modified to reduce different affinity between these two components. In this study, organomodified montmorillonite (Cloisite 15A) was used to improve properties of LDPE/LLDPE blend films. Polypropylene and polyethylene grafted with high percent of maleic anhydride were used as compatibilizer to achieve better interaction between the polyethylenes blend and the organoclay. The aim of this study was to investigate the effect of organo modified MMT on mechanical and gas barrier properties of nanocomposite films of LLDPE/LDPE blends. To investigate the effect of compounding method on properties, nanocomposites were melt-mixed in two ways; 1) materials were double extruded in a twin screw extruder, 2) materials were melt compounded in an internal mixer and then extruded in a twin screw extruder. The feasibility of preparing better exfoliated nanocomposites was studied with using two different compatibilizing agents, PEgMA and PPgMA. After selecting the effective way and suitable compatibilizer, based on XRD and MFI results, nanocomposite samples with various clay contents were prepared and film blown. Mechanical properties of films were studied in machine direction (MD) and transverse direction (TD) (because blown film orients in two directions). Gas permeability of films was also investigated by the constant volume-variable pressure method in a permeability measuring set-up. The XRD and MFI results showed that using the 2 nd way and PPgMA as compatibilizer would make a better dispersion of organoclay in nanocomposite. A broad peak was observed in XRD patterns of the nanocomposite with 1 phr organoclay which indicates partial exfoliation state. The clay diffraction peaks of other nanocomposites shift to lower angles, suggesting intercalated structures in the nanocomposite films. The permeability results showed that addition of organoclay even at low level had significant effect on barrier properties of the nanocomposites. Oxygen permeability decreased by 38% by adding only 4 phr of nanoclay into the blend. Decrease in permeability suggests that barrier properties arise from tortuousity of nanoparticles in the blends. Addition of clay also led to increase in tensile modulus in both directions without decreasing the elongation at break. Nelson’s model and the Halpin–Tsai equation were used to calculate the number of stacks of clay in the nanocomposites using permeability and modulus results, respectively. These models predicted the stack number between 2 and 3 for the nanocomposites, which confirms a high degree of dispersion of Cloisite 15A in the nanocomposites. .