The effects of NiO solid solution into yttria fully stabilized cubic zirconia (YSZ) on the phase stability and electrical properties of YSZ were investigated. Time-dependent conductivity change of NiO-doped YSZ at 1000 o C was also investigated. Raman spectra showed that weak tetragonal peaks in the YSZ monolith disappeared when NiO was added. In addition, the time-dependent conductivity of YSZ monolith sharply decreased after the first 30 h of annealing at 1000 o C, whereas NiO-doped YSZ did not show such decline. Therefore, the solid solution of NiO enhanced YSZ stability and produced time-dependent conductivity change with moderate time.