CaCu3Ti4O12were prepared using conventional solid state reaction processing techniques of ceramic. The CaCu3Ti4O12crystalline phase was verified by XRD patterns. From SEM micrographs, obvious grain and grain boundary structures were observed, and the grain boundary region was comprised of small grains, which were found to be CuO, by EDS, the dielectric constant almost remains at a constant value of 105 in a wide temperature range, but drops dramatically at the frequency of 103 Hz when temperature is below 150 K. The relationship between conductance and temperature of CaCu3Ti4O12 was obtained through the I-V characteristics measured in the temperature range from 93 K to 373 K. It can be separated into three parts with the corresponding activation energies of 0.681, 0.155 and 0.009 eV, which are consistent with the activation energy of CuO ceramics. It was concluded that the small CuO grains in the grain boundary region dominated the DC conductance in CaCu3Ti4O12 ceramics. Thus a new approach for explaining the extraordinary dielectric properties was provided.