A zirconia-yttria-niobia solid solution phase with fluorite structure (F) was prepared as a novel zirconia-based solid electrolyte for oxide fuel cells and oxygen sensors. The phase purity and electrical property of the prepared samples were investigated using powder X-ray diffractometry and two-probe ac conductivity, respectively. The electrical conductivity was studied as a function of temperature, P-O2, and annealing time based on the phase relationship. The extended zirconia system, tri- and penta-valent cations substituted zirconia system, allowed to give a qualitative interpretation on the variation of the conductivity with the composition. In the range of F phase solid solution, it turned out that the activation energy for conduction was the more predominant factor in determining the conduction behavior rather than the concentration of oxygen-ion vacancies. The maximum conductivity could be found near the minimum concentration of the dopant to stabilize fluorite phase (cubic structure).