The permeation of gases through a polymer membrane is a complex process. In most theoretical models a nanocomposite is considered to consist of a permeable polymer phase in which non-permeable nanoparticles are dispersed. The volume fraction of the nanoparticles, their orientation and aspect ratio have strong effects on permeability. One of the purpose of this work was to estimate the aspect ratio of the nanoclays disperse phase using permeability models and also to study the effects of organoclay and compatibilizing agent content (maleic anhydride grafted high density polyethylene, HDPE-g-MA) on the mechanical and transport properties for nanocomposites of high density polyethylene/linear low density polyethylene (HDPE)/LLDPE) blend and organoclay (OMMT). Nanocomposites with 2.5, 5.0 and 7.5 wt% of OMMT at HDPE/LLDPE ratio 3:1 and HDPE-g-MA/OMMT concentration of 2:1 were produced at melt in a twin-screw extruder. Polymer films were prepared using a laboratory scale single screw extruder. The nanocomposites were characterized by X-ray diffraction, tensile testing and water vapor, carbon dioxide and oxygen permeability. The compatibilized HDPE/LLDPE blend-based nanocomposite exhibited an intercalated morphology with a small number of individual platelets dispersed in the HDPE/LLDPE matrix. The nanocomposites behave as a barrier to water vapor and low barrier to O2 and CO2. The clay platelets act as impermeable barriers to the diffusing molecules, forcing them to follow longer and more tortuous paths in order to diffuse through the nanocomposite. Theoretical permeability models proposed by Nielsen, Bharadwaj, Cussler and Aris and Fredickson and Bicerano were used to estimate the aspect ratio of the organoclay particles in the nanocomposites. The models proposed by Nielsen and Bharadwaj allowed to estimate the intercalation state which was supported by the transmission electron microscopy results.