Abstract Nanosize silicon dioxide was used to improve the toughness of cycloaliphatic epoxy resin 3, 4-epoxycyclohexylmethyl-3′,4′-epoxycyclohexane carboxylate. Coupling agent γ-glycidyloxipropyltrimethoxysilane (KH-560) was used to modify the surface property of SiO 2. Impact resistance of the neat epoxy or epoxy/SiO 2 cured system was determined at room temperature by a falling dart impact tester. The distribution of SiO 2 and the fracture morphology of composites were examined using scanning electron microscopy (SEM) after impact testing. The thermal properties of epoxy/SiO 2 composites were studied by thermo-gravimetric analysis (TGA) and differential scanning calorimeter (DSC) analysis. The results showed that the incorporation of nano-SiO 2 effectively improved the toughness of cycloaliphatic epoxy resin as well as its thermal stability.