In this work, two series of nanocomposites of poly(vinylidene fluoride) (PVDF) incorporated with reduced graphene oxide (rGO) and poly(vinyl alcohol)-modified rGO (rGO-PVA) were fabricated using solution-cast method and their dielectric properties were carefully characterized. Infrared spectroscopy and atom force microscope analysis indicated that PVA chains were successfully grafted onto graphene through ester linkage. The PVA functionalization of graphene surface can not only prevent the agglomeration of original rGO but also enhance the interaction between PVDF and rGO-PVA. Strong hydrogen bonds and charge transfer effect between rGO-PVA and PVDF were determined by infrared and Raman spectroscopies. The dielectric properties of rGO-PVA/PVDF and rGO/PVDF nanocomposites were investigated in a frequency range from 10² Hz to 10⁷ Hz. Both composite systems exhibited an insulator-to-conductor percolating transition as the increase of the filler content. The percolation thresholds were estimated to be 2.24 vol % for rGO-PVA/PVDF composites and 0.61 vol % for rGO/PVDF composites, respectively. Near the percolation threshold, the dielectric permittivity of the nanocomposites was significantly promoted, which can be well explained by interfacial polarization effect and microcapacitor model. Compared to rGO/PVDF composites, higher dielectric constant and lower loss factor were simultaneously achieved in rGO-PVA/PVDF nanocomposites at a frequency range lower than 1 × 10³ Hz. This work provides a potential design strategy based on graphene interface engineering, which would lead to higher-performance flexible dielectric materials.