In this study, a conventional model for the resistivity of conductive composites is developed for polymer/graphene nanocomposites taking into account the interphase and tunneling spaces in the conductive networks. The developed model considers the effects of the graphene dimensions, volume fraction of graphene in the conductive networks, contact diameter, number of contacts between nanosheets, orientation angle, interphase thickness, and tunneling distance on the conductivity of the nanocomposites. The experimental results for conductivity and the reasonable effects of different parameters on the conductivity confirm the developed model. Thin and large nanosheets, small tunneling distances, thick interphases, large contact diameters, low orientation angles, and high fractions of percolated nanosheets in the networks can improve the conductivity of a nanocomposite. Moreover, the thickness of the nanosheets and the tunneling distance cause the largest variations in the conductivities of nanocomposites.