Interface design plays a crucial role in developing superior mechanical performance of graphene/polymer nanocomposites. Herein, we report a facile approach to the fabrication of advanced polymeric nanocomposites of epoxy by the incorporation of polyetheramine-functionalized graphene oxide (PEA- f-GO). Two types of PEA molecules with different molecular lengths were used to synthesize the PEA-f- GO sheets. The chemical bonds formed between the amine functional groups on the GO surface and the epoxy resin during curing provided strong sheet/matrix interfacial adhesion. The addition of PEA-f- GO was found to produce significant enhancements in the mechanical properties of epoxy, including elastic modulus, tensile strength, elongation at break and toughness. In particular, the PEA-f-GO sheets containing shorter PEA molecules produced higher improvement in strength but smaller increases in both ductility and toughness than those containing longer PEA molecules. For example, at 0.50 wt% filler loading, two nanocomposites showed increases of 63% and 51% in tensile strength and 90% and 119% in toughness as compared to the unfilled epoxy. Our results suggest that the interphases between the GO and the polymer matrix can be tuned by varying the molecular lengths of grafted modifiers, thereby providing a new route for the rational designing and development of the GO-based composite materials.