Up to 1.5 mol% Nb2O5 was added to 9 mol% Y2O3-stabilized ZrO2. According to an XRD analysis, the addition of Nb2O5 did not change the cubic structure of YSZ. As a result of the repulsion effect of the grain-boundary interface and the preferential segregation of Y2O3, the Nb2O5 segregation at the grain boundaries was not detected by EPMA. The resistances were measured by the complex impedance approach in air, Nb2O5 was found to increase the resistance per unit surface area of the grain boundaries. This result suggested that the most likely mechanism of the dissolution of Nb2O5 in YSZ should be Nb2O5 --> 2Nb(Zr) + 2e’ + 4O(o)(x) + 1/2 O-2. Despite the Nb2O5 addition, the Arrhenius equation can still be applied to the resistances.