Dielectric properties of three different CaCu3Ti4O12 ceramic samples sintered, respectively, in vacuum, air and oxygen are investigated. Three plateaus are detected in the dielectric temperature spectra within 10—300 K for all the three samples, meanwhile the three corresponding peaks of the real impedance and imaginary capacitance occur at a certain temperature. However, the sample sintered in vacuum presents a higher dielectric and clearer real impedance and imaginary capacitance peak, which indicates that oxygen concentration and oxygen vacancy have a great influence on the dielectric property of CaCu3Ti4O12. The results reveal that the three plateaus observed in the dielectric temperature spectra come from the grain, grain boundary and the oxygen vacancy sitting in grain boundary, respectively. The analysis of dielectric spectra indicates that the activation energy of the grain is related to the sintering atmosphere and the oxygen vacancy results in a variable-range-hopping conductivity and polarization for the grain. The activation energy of oxygen vacancy trapper is about 0.46 eV and is nearly independent of sintering atmosphere. The high dielectric constant at low-frequency or high temperature is caused by oxygen vacancy trapping carriers in CaCu3Ti4O12.