A longstanding goal for the varistor community is to understand the evolution of double Schottky barriers during the high-temperature sintering process, which gives varistors their unique nonlinear current-voltage characteristics. Investigating the evolution of dielectric properties in ZnO varistor ceramics during sintering process can provide a more profound insight into their influence on performance. In this paper, negative capacitance is detected as a critical feature in in-situ measurements of capacitance during the sintering of ZnO varistor ceramics. The negative capacitance is observed at high temperatures (∼700 °C) during the heating process. Similar phenomenon is also monitored during the cooling process. Meanwhile, the real part of the capacitance remains constant during the holding periods. The negative dielectric phenomenon conforms to the Drude model, which is essentially caused by the plasma oscillation of electrons. These findings provide a potential approach to construct precise structure function relations of varistor ceramics for further optimization of their performance.