Three-dimensional (3D) graphene (GR) network/phenolic resin composites with tunable and weakly negative permittivity were prepared by mechanical mixed method. Dielectric properties including permittivity, dielectric loss tangent and alternating current conductivity ( σ ac ), were investigated in detail. When the GR content was increased from 8 to 13 vol%, a ladder-shape increase of σ ac was observed, indicating a percolation phenomenon. It was found that, when the GR content exceeded 13 vol% the negative permittivity appeared attributed to the formation of 3D interconnected GR network. Moreover, the negative permittivity was easily adjusted by controlling the GR content. Compared with the metal-ceramics in our previous work, the absolute values of negative permittivity were low, which was in favor of impedance matching. For the GR content of 13 vol%, high dielectric loss above 25 over the whole frequency was also observed. Finally, the equivalent circuits were used to analyze the reason of the negative permittivity. [ABSTRACT FROM AUTHOR]