Polymer-layered silicate nanocomposites are considered to be of technological importance since they exhibit unique physical and mechanical properties. The polymer nanocomposites reinforced with low loading levels of montmorillonite (MMT) (2-4 wt%) were prepared by melt-compounding. The matrices of nanocomposites were polar polyamide-6 (PA6) and non-polar polyolefins, i.e polypropylene (PP) and high density polyethylene (HDPE). The intercalation of molecular chains into the galleries of MMT depended greatly on the polarity of polymer matrices and the organic surfactants applied to the clays. For polyolefins, homogenous dispersion and delamination of the silicate layers in the polymeric matrices were difficult to achieve. In this regard, a compatibilizer functionalized with maleic anhydride (MA) was used to facilitate the intercalation of polyolefin molecular chains into the MMT galleries. The structure, morphology, crystallization and mechanical properties of the polymer-clay nanocomposites were stud ied by means of the X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), tensile and impact measurements. XRD traces revealed that PA6 crystallized exclusively in g -crystalline structure with the incorporation of MMT. And the PA6-MMT nanocomposites exhibited an intercalated structure as evidenced by the shift of (001) diffracting peak of MMT to lower angle region. TEM examination revealed the formation of a mixed intercalated/exfoliated structure for the PA6-MMT nanocomposites. A similar mixed intercalated/exfoliated structure was observed in MA compatibilized PP-MMT nanocomposites. DSC results showed that the additions of MMT into polymers resulted in an increase of the crystallization peak temperature and a decrease of the degree of crystallinity. This implied that MMT layers act as heterogeneous sites for the nucleation of crystallites and physical obstacles to the motion of molecular chains during crystallization process. Tensile and impact measurements showed that the PA6-MMT, PP-MMT and HDPE-MMT nanocomposites exhibit high yield strength but much lower tensile ductility and impact toughness compared to their parent homopolymers. To enhance the tensile ductility and impact toughness, elastomer particles such as styrene-ethylene-butylene-styrene grafted with maleic anhydride (SEBS-g-MA) were incorporated into these nanocomposites. The high polarity of MA functional group of SEBS-g-MA tended to enhance the compatibility between the clay and nonpolar polymers. Mechanical measurements demonstrated that the elastomeric particles are effective to restore the tensile ductility and impact strength of the nanocomposites at the expense of their tensile strength and stiffness. Accordingly, the essential work of fracture (EWF) concept under tensile and impact conditions can be used to characterize the fracture toughness of the polymer nanocomposites investigated. Tensile EWF results showed that elastomer particle additions were beneficial in enhancing the essential and non-essential work of the nanocomposites investigated. Impact EWF measurements indicated that the SEBS-g-MA elastomers increased the fracture toughness of the nanocomposites. SEM observations revealed that cavitation of elastomer particles and its associated shear yielding of polymer matrix were the main energy dissipation mechanism for the nanocomposites. From the results of structural and mechanical properties of nanocomposites investigated, the correlation between the structure and properties of nanocomposites is discussed.