The electrical properties of L285 epoxy composite materials (CMs) with a mixed filler including graphite nanoplatelets (GNP)/barium titanate(BT) at various GNP concentrations and at 35 wt.% of BT are measured using the AC impedance spectroscopy with frequencies ranging from 1 kHz to 2 MHz. The complex permittivity and its frequency dependence increase with the GNP content. The addition of a BT filler in GNP/L285 CM increases the percolation threshold in addition to a slight increase in the permittivity, allowing a higher GNP content in the composite without the formation of a conductive network. The AC conductivity increases and its dependence on the frequency decreases, as the GNP content increases.