An anomalous positive temperature coefficient (PTC) of resistance was found in a conductor–insulator ceramic composite. The composite materials was composed of randomly dispersed conducting graphite and insulating cristobalite‐form SiO2particles. The PTC effect occurred at exactly 230°C, that is, at a reversible phase inversion point with a large thermal expansion of cristobalite. The magnitude of the PTC anomaly was high in composites located in the middle of the percolation curve, and loaded with large graphite and fine SiO2parti