The influences of loading rate (0.1–1000 mm/min) and fumed-silica weight fraction (0–3 wt%) on mechanical propertiesand fracture toughness of fumed silica/epoxy nanocomposites have been investigated. The fracture toughness (KIQ) was influenced by loading rate and amount of fumed-silica particles, i.e., they increased with decreasing loading rate and increasing fumed-silica weight fraction. The increase of fumed-silica weight fraction enhanced the mobility of polymer chains, reduced the glass-transition temperature, and improved the ductility and fracture toughness of the present nanocomposites. Localized-inelastic deformation (shear lip, stretched zone, crazing, and crack blunting) and planestress condition at crack tip were dominating mechanisms at low loading rate (0.1 mm/min). On the other hand, less localized-inelastic deformation and plane-strain condition at crack tip were dominating mechanisms at high loading rates (10–1000 mm/min).