The dielectric properties of Zr substituted CaCu3Ti4O12 ceramics have been investigated in detail. Grain size decreases with Zr content increasing. The hetero-electrical microstructures of prepared samples have been confirmed by the impedance spectra. The dielectric loss has been improved by Zr doping because of the enhancement of grain boundary resistivity. A Debye-like boundary relaxation behavior has been observed in the temperature range of 220-600K. As Zr content increases, the relaxation time increases due to the higher grain boundary concentration. This work has provided an additional proof for the origin of giant dielectric response in CaCu3Ti4O12 ceramics.