Low-density polyethylene (LDPE)- and linear low-density polyethylene (LLDPE)-based polymer nanocomposites (PNC) containing organically modified montmorillonite (MMT) were prepared by melt mixing process. Itaconic acid (IA) and monomethyl itaconate (MMI) grafted polyethylene (PE) were used as compatibilizers. Na-MMT was modified by dodecyl-, hexadecyl-, and octadecyl-ammonium chlorides and then the compatibilizers were synthesized in solution reaction. 5 wt% organoclay and different concentration of compatibilizers (5, 10, and 15 wt%) were used for the PNC preparations to compare the effects of compatibilizers. The interlayer spacings, d(001), and the mechanical properties of samples were measured by XRD analysis and universal testing machine, respectively. The experimental tensile moduli were used in Halpin-Tsai micro mechanical model to obtain "l/t" ratios and the results were compared experimental d(001) values obtained from XRD data. It was found that the experimental results are in good agreement with the calculated results.