The surface of graphene oxide (GO) was covalently modified by two kinds of monofunctional poly(oxypropylene)amines with different molecular chain lengths. Thermogravimetric analysis and other relevant analysis confirmed that both monoamine molecules were successfully grafted onto GO nanosheets. The monoamine molecules with longer molecular chain functionalized GO showed better compatibility and higher degree of exfoliation in the epoxy matrix, achieving better enhancement of both strength and elongation at break. The tensile strength, elongation at break, flexural strength and dielectric constant of the epoxy nanocomposite exhibit 29, 77.9, 28.5 and 6000% (0.1 Hz) increase at 0.05 wt% loading fraction when compared with unmodified counterparts, respectively. Our results demonstrate a great potential for industrial applications.