The incorporation of nano-sized particles into epoxy has received increasing attention in recent years as a means of improving the mechanical performance of continuous fiber reinforced epoxy composites. Developments in surface chemistry have allowed high concentrations of spherical nanosilica particles to be uniformly dispersed in epoxide without agglomeration, filtration, or excessive increase in mix viscosity. Until the present time, however, only limited mechanical performance data have been published on such highly dispersable nanosilica in high temperature epoxies, especially in resin systems with high glass transition temperatures. In the current research, surface modified nanosilica in concentrations of up to 34.4% by weight in a diglycidyl ether of bisphenol F epoxide with 3,3' or 4-4' diamino diphenyl sulfone curing agents was investigated to determine the resulting mechanical properties. It was observed that Mode I fracture toughness and modulus improved significantly on addition of nanosilica without significantly affecting the glass transition temperature. Although the density of the resin system increased with increasing nanosilica concentration, specific modulus and specific fracture toughness also improved. Based on the experimental results, micromechanical models were used to back-calculate plausible elastic moduli of assumed interphase regions between the NS particles and the epoxy.