The three different sized chemical functionalized graphene (GO) sheets, namely GO-1 (D50 = 10.79 μm), GO-2 (D50 = 1.72 μm) and GO-3 (D50 = 0.70 μm), were used to fabricate a series of epoxy/GO nanocomposites. Fracture toughness of these materials was assessed. The results indicate that GO sheets were dramatically effective for improving the fracture toughness of the epoxy at a very significant low loading. The enhancement of the epoxy toughness was strongly dependent on the size of GO sheets incorporated. GO-3 with smaller sheet size gave the maximum reinforcement effect compared with GO-1 and GO-2. The incorporation of only 0.1 wt% GO-3 was observed to increase the fracture toughness of pristine epoxy by ∼75%. The toughening mechanism was well understood by fractography analysis of the tested samples. Massive cracks in the fracture surfaces of the epoxy/GO nanocomposites were observed. The GO sheets effectively disturbed and deflected the crack propagation due to its two dimensional structure. GO-3 sheets with smaller size were highly effective in resisting crack propagation, and a large area of whitening zone was observed. The incorporation of GO also enhanced the stiffness and thermal stability of the epoxy.