CaCu 3 Ti 4 O 12 (CCTO) ceramic powders were successfully prepared by a wet-chemical combustion method. The phase formation, microstructure, giant dielectric response, and nonlinear electrical properties of the sintered ceramics were systematically investigated. The main phase in the CCTO powder is clearly indicated in the XRD pattern. A dense and fine-grained microstructure was obtained by sintering the compacted CCTO powders. Dielectric permittivity values are in the range of ~ 10 3 –10 4 with a very low tan δ of ~ 0.03–0.11 at 1 kHz. Interestingly, a high breakdown electric field (9741.6 V/cm) and nonlinear coefficient (9.9) are observed in the CCTO ceramic sintered at 1030 °C for 1 h. Impedance spectroscopy analysis revealed that the electrical conductivity in the grains and grain boundaries is completely different. The electrically heterogeneous microstructure clearly indicates that the giant dielectric response and nonlinear electrical properties are correlated with the electrical response of the insulating grain boundaries. The origin of the n -type semiconducting grains is evident by considering the oxidation states of the Ti and Cu ions, which are quantitatively and qualitatively analyzed using X-ray absorption spectroscopy.