Covalent modification of montmorillonite (MMT) with 3-amino propyl triethoxy silane followed by intercalated modification by tributyl tetradecyl phosphonium chloride was conducted to produce double-modified MMT. The obtained double-modified MMT was compounded with polypropylene (PP) to obtain nanocomposites by melt blending. The dispersion of the double-modified MMT in PP was greatly improved by the addition of maleic anhydride-grafted-polypropylene (PP-g-MA) in PP matrix. PP-g-MA acted as a “compatibilizer”, whose anhydride groups form chemical bonding with the amino groups on the surface of the MMT sheets , which increases the dispersion of MMT in PP. Fourier transform infrared, X-ray diffraction, transmission electron microscopy, thermal gravimetric analysis and scanning electron microspocy were used to characterize the structure, morphology, and thermal properties of the nanocomposites. The results show that PP-g-MA promotes the formation of exfoliated/intercalated morphology and impart better thermal properties in air to PP filled with the double-modified MMT, compared with PP/MMT composite without PP-g-MA.