Electrical percolating composites of polypropylene (PP) filled with five different graphene nanoplatelet (GNP) fillers and their hybrid systems were prepared using melt blending. The effect of GNP size and their hybrid system on the conductive network formation is investigated. The formation of a conductive network can be affected by the structure and morphology of GNPs of different sizes. The GNPs with a larger diameter and smaller thickness are beneficial to produce a conductive network. The conductivity of the PP/GNP composite depends on the aspect ratio of the GNPs when the content exceeds the percolation threshold. However, when the GNP content is near the percolation threshold, both diameter and dispersion of the GNPs can affect the conductivity significantly, and electron tunneling theory should be taken in account. The highest electrical conductivity was obtained for a PP/large-diameter GNPs/medium-diameter GNPs hybrid system. To explain the hybrid system, an "island-bridge"-structured conductive network is proposed. The better conducting network may be due to scattered "islands" that connect with each other via a long "bridge." This bridge links the islands for better charge transport across the GNPs and the obstruction of PP matrix, which enables the formation of a better conducting network. Even though GNPs with small diameter show perfect dispersion, they contribute less to the formation of a conductive network.