Our efforts were focused on improving thermal stability of polyethylene (PE) and polypropylene (PP) through incorporation of new organically modified montmorillonite. The reactive organofilic montmorillonite (MMT-EA) was prepared by exchanging the interlayer inorganic cations in montmorillonite by a large excess of ethyl 2-aminobenzoate (EA). Organic modification can affect not only microstructures, but also the properties of the polymer - montmorillonite nanocomposites. In the next step novel polyolefins/ organoclay nanocomposites were prepared by melt intercalation process. In order to obtain this new family of materials and enhance their properties was used very low filler level, from 1.5 to 5 wt.%. The dispersion of the montmorillonite layers in the polymers was characterized by using X-ray diffraction (XRD). The thermal properties and structure changes were studied by different scanning calorimetry (DSC) and thermogravimetric analysis (TGA). Further studies indicated that the exfoliated clay modified with EA improved thermal stability of the PE and PP/MMT-EA nanocomposites.