In this paper, graphene oxide (GO) was functionalized with a hyperbranched cyclotriphosphazene polymer, which was synthesized by the repeated reactions of hexachlorotriphosphazene with hexamethylenediamine. Subsequently, the resultant functionalized GO was incorporated into dicyclopentadiene bisphenol dicyanate ester (DCPDCE) to prepare composites. Fourier-transform infrared spectra, X-ray photoelectron spectroscopy, X-ray diffraction and transmission electron spectroscopy were employed to examine the surface functionalization of GO. The effects of functionalized GO on the curing reactivity, mechanical, dielectric, thermal and water resistent properties of DCPDCE resin were investigated systematically. Results show that the addition of modified GO can facilitate the curing reaction of DCPDCE. Meanwhile, the appropriate content of modified GO can enhance the mechanical properties including impact and flexural strengths of DCPDCE resin. When modified GO content is 0.6 wt%, the corresponding composite exhibits slightly higher dielectric constant but lower dielectric loss than pure DCPDCE resin over the testing frequency from 10 to 60 MHz. In addition, the thermal stability and moisture resistance of modified GO/DCPDCE nanocomposties are also superior to that of pure DCPDCE resin.