The change in conductivity of 8 mol% and 10 mol% yttria stabilized zirconia (8YSZ and 10YSZ, respectively) were measured during continuous current loading at 1000°C. The conductivity of 8YSZ gradually degraded with holding time in the test and it decreased to about 65% of the initial value after annealing for 1500h. However, the conductivity of 10YSZ is constant under the same condition. The crystal structure of the samples before and after annealing was investigated by X-ray diffraction techniques, Transmission electron microscopy, and Raman spectroscopy. It was concluded that the deterioration in conductivity of 8YSZ related to the phase transformation, such as gradual formation of fine tetragonal phase which had lower conductivity than cubic phase had.