Functional graphite oxide (DGO) with amine-rich surface was synthesized through chemically grafting flexible poly(oxypropylene)diamine, and its epoxy (EP) composites were prepared. Fourier transform infrared spectra (FTIR) etc. confirmed the realization of chemical functionalization of DGO. The covalent functionalization of graphite oxide (GO) with poly(oxypropylene)diamine was favorable to its homogeneous dispersion in epoxy matrix. Meanwhile, the strong covalent interface formed between epoxy and DGO promoted the stress transfer. The addition of 0.3 wt% DGO increased the tensile strength, flexural strength, elongation at break and toughness of the epoxy resins by 20%, 40%, 90% and 145%, respectively. This showed higher improvements than those addition of GO. Therefore, significant improvements both in the strength and toughness of epoxy nanocomposites were achieved by the addition of trace DGO.