Zn-substituted CaCu3Ti4O12 ceramics were synthesized by solid-state sintering. Their microstructures and dielectric properties were investigated. Ca(Cu1-xZnx)(3)Ti4O12 single-phase structures were obtained up to x = 0.1, and the Cu+/CU2+ and Ti3+/Ti4+ mixed-valent structure was enhanced with increasing Zn substitution. The giant dielectric response was significantly enhanced by Zn substitution. The dielectric constant increased with increasing x, and a giant dielectric constant plateau as high as similar to 9 x 10(4) was achieved for x = 0.1 at 10 kHz, while that for x = 0 was similar to 3 x 10(4). The enhanced giant dielectric response was profoundly concerned with the modified mixed-valent structure.