In this paper, we report the preparation and dielectric properties of reduced graphene oxide/polypropylene (rGO/PP) composites with an ultralow percolation threshold as low as 0.033 vol%. This value is the lowest among those that have been reported in graphene-filled composites. The rGO/PP composites were prepared through a latex technique, which consists of an in-situ chemical reduction of graphene oxide in PP latex and a subsequent filtration. Scanning electron microscopy and X-ray diffraction measurements demonstrate that the homogeneous dispersion of rGO nanosheets in the PP matrix was realized. A blue shift in Raman G band of the rGO nanosheets was observed in the rGO/PP composites, indicating the strong interaction between the rGO filler and the PP matrix. In the frequency range from 10(2) Hz to 10(7) Hz, the rGO/PP composites showed an insulator-to-conductor percolation transition as the increase of the rGO loading. Near the percolation threshold, the dielectric permittivity of the rGO/PP composites underwent a significant change of three orders of magnitude. Moreover, the permittivity was found to be temperature dependent. (c) 2013 Elsevier Ltd. All rights reserved.