High density polyethylene- and polypropylene-clay nanocomposites are synthesized by melt blending, in which polyethylene glycol and polypropylene glycol are used as compatibilizers to increase the space of galleries. The morphology properties of nanocomposites are explored by X-ray diffraction and transition electron microscopy. The thermal conductivity coefficient (K) of nanocomposites is also measured along with the thermal stability. A conventional model based on developed Maxwell-Garnett formula is also established to predict the thermal conductivity of polymer/clay nanocomposites with clay loading. Morphology results indicate that two intercalated and exfoliated structures are formed. The established model satisfactorily predicts the K values of nanocomposites for low range of clay content. Thermogravimetric analysis shows remarkable thermal stability of nanocomposites with 10 wt % of clay content. The deviation of our model from experimental result for 10 wt A of clay can be attributed to the intercalated structure of layered silicates into the matrices. Although the K values do not considerably increase in 5 wt A with respect to the increase occurs for 10 wt A of clay, but it increases about 28 and 37% at 50 degrees C for high density polyethylene- and polypropylene-clay nanocomposites, respectively. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 118: 1042-1050, 2010