Conductivity degradation behavior of NiO doped yttria stabilized zirconia (YSZ) was examined by AC impedance technique under an OCV condition with a test cell consisting of air, Pt/NiO doped YSZ/Pt, 1% H2O-H-2, at 1173 K. Conductivity gradually decreased parabolically, and reached half of its original value after 170 h testing. Cross section analysis of the tested electrolyte revealed that color changed into dark from the anode side surface to inside the electrolyte. Micro-Raman analysis indicated the phase transformation of NiO doped YSZ from the cubic phase to the tetragonal phase in the area where color changed into dark. It has been concluded that conductivity degradation is induced by the phase transformation of zirconia and that such phase transformation is dominantly governed by the oxygen potential distribution inside the electrolyte. (C) 2012 Elsevier By. All rights reserved.