Percolation, positive temperature coefficient (PTC), and negative temperature coefficient (NTC) in carbon black-filled HDPE composites, were studied by measuring AC electrical properties. An equivalent single parallel circuit model, which consists of a resistor and a capacitor, was adapted to explain the distribution of carbon black in composite. The electrical conductivity and admittance and phase angle were characterized by measuring the distribution of carbon black aggregates with an impedance analyzer. It was concluded that PTC is due to the deagglomeration or the breakage of the conduction networks caused by thermal expansion and/or crystalline melting of the matrix polymer, and NTC is due to the reagglomeration of the network of carbon black aggregates.