The nanocomposites with inorganic nano powder, improved thermal stability, were prepared by urethane polymerization. The structure and surface properties of the nanocomposites were determined by X-ray diffraction and FT-IR, respectively. The thermal stabilities were studied using TGA and DSC. Their morphologies and mechanical properties were observed by SEM and UTM. As a result, the nanocomposites with MMT led to the increase of the silicate layers. The distance between layers of the nanacomposites with MMT was increased by $7.5{AA}$ and the new peaks at $1038cm^{-1}$ were shown in the presence of the Si-O groups on the silica. The thermal stabilities of the nanocomposites were higher than those of pore polyurethane matrix. The nanocomposites had higher in mechanical properties than the pure polyurethane matrix.