PET does not create a direct hazard to the environment, but due to its substantial fraction by volume in the waste stream and its high resistance to the atmospheric and biological agents, it is seen as a noxious material. Unsaturated polyester (UP) resin based on recycling PET is widely used for the matrix of composites such as fiber reinforced plastic (FRP) and polymer concrete (PC). Consequently, inexpensive and high performance resins are important for the future of polymer composites. One recent method for enhancing the performance of polymer composites are the manufacture of MMT (montmorillonite)-UP nanocomposite synthesized by intercalating the UP resin into the silicate layers of MMT. This study investigates the mechanical and thermal properties of UP-MMT nanocomposites. The structures of UP-MMT nanocomposite were investigated by X-ray diffraction and transmission electron microscopy (TEM). Test results indicate that the mechanical, barrier and thermal stability properties of UP-MMT nanocomposites are better than those of pure UP. Improved performance of UP is very important for polymer concrete.