Al2O3 nanoparticles have been successfully synthesized by hydrothermal process. Subsequently, epoxy (EP) nanocomposites incorporating the Al2O3 nanoparticles have been fabricated. The morphologies of the Al2O3 nanoparticles and the nanocomposites were characterized by scanning electron microscopy (SEM). Dielectric properties of these nanocomposites are strongly related to both the morphology of the filler and the dispersion quality. Thermogravimetric analysis (TGA) reveals an enhanced thermal stability of the EP/Al2O3 nanocomposites as compared to that of the pure EP due to the strong interaction between the nano fillers and the polymer matrix.