Oxide ion conductivity and internal friction were examined for yttria and ceria co-doped cubic zirconia solid solution. The maxima of the internal friction (IF) were observed for Zr0.8 - xCexY0.2O19 (x = 0,03, 0.5) polycrystals in the temperature range of 300-680 K at 3.2 Hz using forced torsion method. The CeO2 addition reduced both the oxide ion conductivity in low temperature range and IF peak amplitude of Zr0.8 - xCexY0.2O1.9. The possible structural relaxation is proposed to decrease ionic conductivity by increasing Ce concentration in Zr-0.8 (-) xCexY0.2O1.9 even with the same vacancy concentration. (C) 2013 Elsevier B.V. All rights reserved.