Doped ceria (CeO 2 ) compounds are fluorite type oxides which show oxide ionic conductivity higher than yttria stabilized zirconia, in oxidizing atmosphere. As a consequence of this, considerable interest has been shown in application of these materials for 'low temperature operation (500 -650 8C)' of solid oxide fuel cells (SOFCs). In this study, Y x Ce 12x O 22d (x ΒΌ 0:05; 0:1; 0:15; 0:2 and 0.25) fine powders were prepared using a carbonate co-precipitation method. The relationship between electrolytic properties and nano-structural features in the sintered bodies was examined. The micro-structures of Y 0.05 Ce 0.95 O 1.975 , Y 0.15 Ce 0.85 O 1.925 and Y 0.25 Ce 0.75 O 1.875 as representative three specimens have been investigated in more detail with transmission electron microscopy (TEM). The big diffuse scattering was observed in the background of electron diffraction pattern recorded from Y 0.15 Ce 0.85 O 1.925 and Y 0.25 Ce 0.75 O 1.875 sintered bodies. This means that the coherent micro-domain with ordered structure is in the micro-structure. While Y 0.25 Ce 0.75 O 1.875 sintered body with low conductivity and high activation energy has big micro-domains, the micro-domain size in Y 0.15 Ce 0.85 O 1.925 with high conductivity and low activation energy was much smaller than that of Y 0.25 Ce 0.75 O 1.875 . TEM observation gives us message that the size of coherent micro-domain with ordered structure would closely relate to the electrolytic properties such as conductivity and activation energy in the specimens. It was concluded that a control of micro-domain size in nano-scale in Y 2 O 3 doped CeO 2 system was a key for development of high quality solid electrolyte in fuel cell application.