The structural, morphological, and dielectric properties of CaCu3Ti4O12 (CCTO) ceramics without and with 10% HfO2 by weight were investigated in this work. The main effect of adding HfO2 on the microstructure was the segregation of HfO2 subgrains within CCTO grains, which consequently causes the inhomogeneous electric structure of the HfO2-modified sample. The HfO2-modified sample presented the much larger dielectric constant than the pure CCTO one, which resulted from HfO2 subgrains' effect as another new origin of the dielectric constant. (C) 2012 The Electrochemical Society. [DOI: 10.1149/ 2.089204jes] All rights reserved.