The ac conductivity and dielectric properties of CaCu3Ti4O12 (CCTO) ceramics were investigated in a temperature range of －120℃ to 300℃ and a frequency range of 1 Hz to 10 MHz. Two different conduction processes, which can be well described by Mott’s variable-range-hopping (VRH) mechanism, were observed in different temperature regions. The high temperature VRH conduction is related to the second ionization of oxygen vacancy. The low temperature dielectric properties of CCTO could be described by the so-called universal dielectric response (UDR) when a polaron relaxation is considered.