The influence of a Nb2O5 additive on the densification, microstructural morphology, and nonlinear electrical properties of the CuO-doped SnO2-based varistors was investigated. It was found that copper oxide significantly improves the densification of the SnO2 ceramics. The effects of Nb2O5 on tin dioxide varistors were analyzed by X-ray diffraction, scanning electron microscopy, energy-dispersive spectroscopy, and capacitance-voltage measurements, as well as impedance spectroscopy. Copper oxide segregates at the grain boundaries and precipitates at triple points, and niobium makes the tin dioxide grain semi-conductive. The copper oxide intergranular insulating layer separates two semi-conductive tin dioxide grains and forms the barriers. The reason for the nonlinearity of SnO2-based ceramics was explained.