A hybrid nanocomposite was prepared from the combination of an epoxy resin with a dispersion of nanometer-sized silica particles prepared by hydrolysis of silicon tetra alkoxides A silane coupling agent was used to improve the filler-matrix interface by crosslinking into the epoxy networks. Silica dispersion caused the increase of nanometer size voids evaluated by the positron annihilation method and the decrease of Vickers hardness. The use of coupling agent also brought about the decrease of nanometer size voids and the increase in hardness.