Nanocomposites formed from blends of high density polyethylene (HDPE) and maleic anhydride-grafted high density polyethylene (HDPE-g-MA) and M-2(HT)(2) organoclay were melt processed to explore the extent of exfoliation and the mechanical properties. Wide angle X-ray scattering (WAXS) and transmission electron microscopy (TEM) coupled with detailed particle analysis were used to determine the effect of HDPE-g-MA content and organoclay content on exfoliation and mechanical properties. As the HDPE-g-MA content increases, the global average particle aspect ratio initially increases drastically, reaches a maximum, and slightly decreases. The fraction of single platelets, however, increases at a steady rate for nanocomposites with HDPE-g-MA contents >= 25%. Relative modulus initially improves with increased levels of HDPE-g-MA, and then levels off with greater HDPE-g-MA content. Izod impact strength reaches a maximum at low HDPE-g-MA levels, decreases below the value for the pure HDPE nanocomposite, and levels off at higher HDPE-g-MA content. A composite model based on the Mori-Tanaka theory was developed to treat organoclay tactoids and single platelets as two separate types of fillers. This model gives rather good quantitative agreement between the predicted values of modulus calculated from the TEM results and that measured experimentally. (C) 2010 Elsevier Ltd. All rights reserved.