CaCu3Ti4O12 ceramic with a giant dielectric constant was synthesized by sol-gel method and sintered in three different sintering conditions: 1 035 °C for 48 h, 1 080 °C for 3 h and 48 h. The phase of the ceramics, the element distribution, the valance state of Ti ions at grain boundaries, and the electrical properties were characterized via X-ray diffraction (XRD), energy dispersive X-ray analysis (EDAX), X-ray photoelectron spectroscopy (XPS), electrical conduction and dielectric measurement. The results demonstrate that the grain-boundary microstructure and the electrical properties are influenced by sintering conditions: ➀ By raising sintering temperature, the Cu-rich and Ti-poor grain boundary was formed and grain resistivity was decreased. ➁ By prolonging sintering time, the content of Ti3+ near the grain boundary increased, leading to the decrease of the grain-boundary resistivity and the increase of the activation energy at grain boundary. The ceramic, sintering at 1 080 °C for 48 h, exhibited a small grain resistivity (60.5 Ω · cm), a large grain-boundary activation energy (0.42 eV), and a significantly enhanced dielectric constant (close to 1×105 at a low frequency of 1×103 Hz ). The results of electrical properties accord with the internal boundary layer capacitor model for explaining the giant dielectric constant observed in CaCu3Ti4O12 ceramics.