The effect of fumed silica weight fraction (1%, 3%, 5%, and 15%) on the mechanical properties of a polymer nanocomposite was investigated. The composite was fabricated by dispersion of 12 nm spherical silica in Epon 862 by ultrasonication. Uniaxial tension and single edge notched tension (SENT) fracture specimens were tested with a custom-made tensile testing apparatus. Stress-strain curves were calculated by applying digital image correlation (DIC) on optical images. The composite modulus increased monotonically with weight fraction. On the other hand, the ultimate strength was rather insensitive to weight fraction. The critical mode I stress intensity factor increased with silica weight fraction, becoming as high as 35% compared to the neat epoxy for 15 wt.% silica composites. Electron microscopy fractographs showed that the nanoparticles induced micro-flake morphologies to the fracture surface of 15% composites, which contributed to matrix toughening by enhancing matrix yielding. Voids of the size of the nanoparticles were also observed, but their extent was limited compared to surface roughening due to micro-flakes.