The effect of ozone surface treatment of montmorillonite (MMT) was investigated in thermal stabilities of polypropylene (PP) nanocomposites. Sodium montmorillonite (Na$^$+/-MMT) was organically modified with dodecylammonium chloride. The surface properties of MMT, including the specific surface area (S$\_$BET/), equilibrium spreading pressure ($pi$$\_$e/), and London dispersive component (${gamma}$s$^$L/), were studied by the BET method with $N_2$ adsorption. Also, the thermal stabilities of the nanocomposites were investigated in DSC and TGA. As experimental results, $pi$$\_$e/ and ${gamma}$s$^$L/ of the ozonized dodecylammonium chloride (DA-MK ( $O_3$)) were increased in about 1.7 and 3.5 mJ/ $m^2$, resulting from the increasing of the micropores. From the DSC results, it was found that the melting temperature and crystallization temperature of PP/DA-MK and PP/DA-MK ( $O_3$) were higher that those of pure PP. These results were explained that dodecylammonium chloride of nano-scale led to a nucleation effect for PP crystallization. Also, it was found that E$\_$t/ of the PP/DA-MK ( $O_3$) nanocomposies was increased within about 64 kJ/mol. These results were probably explained by the improvement of dispersivity of DA-MK ( $O_3$) in a PP matrix.