A composite system of CaCu3Ti4O12/CaTiO3 doped with Zr4+ was deliberately created using a one-step processing method. Investigation of the microstructural evolution and electrical responses of internal interfaces of Zr4+-doped CaCu3Ti4O12/CaTiO3 composites was performed to clarify the exact nature of its high dielectric response. Grain sizes of the CaCu3Ti4O12 phase in the CaCu3Ti4O12/CaTiO3 microstructure were largely increased by doping with Zr4+, resulting in an increase in epsilon' (at 10 Hz) from 1.86 x 10(3) to 1.03 x 10(4). This is in complete contrast to observations of a single phase of Zr4+-doped CaCu3Ti4O12. This observation confirmed an extrinsic effect as the origin of high dielectric properties of CaCu3Ti4O12/CaTiO3 composites, rather than intrinsic factors. The macroscopic dielectric relaxation was well described by the Maxwell Wagner relaxation model. Furthermore, changes of the loss tangent resulting from different doping levels of Zr4+ correlated well with variation of the resistance of insulating internal interfaces. Experimental results gave an important clue indicating that electrical responses of internal interfaces were the cause of the giant dielectric response in the CaCu3Ti4O12 material system. (C) 2014 Elsevier Ltd. All rights reserved.