Reduced graphene oxide (RGO)/Co3O4 nanohybrid particles, composed of reduced graphite oxide and Co3O4 particles, have been fabricated by an in situ growth method under mild wet-chemical conditions (140 °C). A series of characterization results indicate that the as-prepared Co3O4 particles with relatively uniform sizes are embedded in RGO layers to form unique core–shell nanostructures. The RGO/Co3O4/poly(vinylidene fluoride) composite was found to possess excellent absorption properties. Owing to the effect of the negative permeability, the position of the absorption peaks remains at the same frequency at different thicknesses without shifting to lower frequencies. For the composites with a filler loading of 10 wt %, the maximum peaks can reach −25.05 dB at 11.6 GHz with a thickness of 4.0 mm. These enhanced microwave absorbing properties can be explained based on the structures of the nanohybrid particles.