Four high purity heavily yttrium doped ceria (YDC, YxCe1-xO2-delta, x=0.1, 0.15, 0.2 0.25) electrolytes were prepared and their grain boundaries (CBs) were investigated by impedance spectroscopy (IS) and transmission electron microscopy (TEM). IS data indicated that single GB's resistance and GB's thickness firstly decreased and then increased with increasing doping level. TEM investigation on CBs suggested the formation of nanodomains at CBs was responsible for the increase of GB's electrical thickness. Our work suggests in heavy doping level ceria, in addition to the space charge effect, nanodomain's blocking effect is another contribution to the high GB resistivity. According to the microstructure evolution of YDC with increasing doping level, we suggest three factors that influence nanodomain's segregation to CBs. (C) 2012 Elsevier B.V. All rights reserved.