We achieved for the first time the formation of a charge-transfer complex (CTC) between a novel hyperbranched polyimide (PI) and oligo-imide-functionalized graphene oxide (FGO), aiming for enhancing the dielectric properties of resulting PI-FGO nanocomposites. This novel hyperbranched PI was derived from a new diamine N-1,N-1'-(4,4'-oxybis(4,1-phenylene))bis(N-1-(4-aminophenyl)benzene-1,4-diamine). The imide moieties were integrated on amine-FGO via a step-by-step condensation and thermal imidization approach. This FGO exhibited excellent compatibility with hyperbranched PI because of the formation of a CTC between two domains. In viscoelastic measurements, the dynamic storage modulus and glass-transition temperature of flexible PI-FGO nanocomposites increased linearly with increasing FGO contents. The synthesized nanocomposites revealed high mechanical properties with a tensile strength as high as 1.122 GPa. Thermogravimetric analysis demonstrates that these nanocomposite films exhibit high thermal stability up to 550 degrees C. Remarkably, the dielectric constant increases up to 42.47 at 8 wt % FGO loading with a dielectric loss as low as 0.0018 while maintaining the breakdown strength as high as 147.3 +/- 4.5 MV/m.