Polypropylene (PP)/clay nanocomposites (PPCN) were prepared via grafting-melt compounding by using a new kind of co-intercalation organophilic clay which had a larger interlayer spacing than the ordinarily organophilic clay only modified by alkyl ammonium. One of the co-intercalation monomers was unsaturated so it could tether on the PP backbone by virtue of a grafting reaction. The larger interlayer spacing and strong interaction caused by grafting can improve the dispersion effect of silicate layers in the PP matrix, which was confirmed by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The mechanical properties of PPCN were improved with the addition of clay. The incorporation of silicate layers also gave rise to a considerable increase of the storage modulus (stiffness) and a decrease of the tan delta value, demonstrating the reinforcing effect of clay on the PP matrix. Besides, the glass transition temperature (T-g) of PPCN decreased in the presence of the silicate layers. The addition of clay did not change the crystal structure of PP, however silicate layers acted as nucleating agents for the crystallization of PP.