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CaCu 3-x Zn x Ti 4 O 12 (x is from 0 to 1?0) polycrystalline samples were fabricated via a two-step solid state reaction process. The lattice parameter of the monophasic CaCu 3 Ti 4 O 12 phase increased as Zn content increased. Scanning electron microscopy (SEM) images of the CCTO ceramic show bimodal grain size distribution and the grain size decrease largely with the appearance of Zn 2 TiO 4 second phase. The dielectric permittivity of pure CCTO ceramic is ,1?5610 4 at f5100 Hz. The dielectric constant of the sample largely increased with Zn substitution in the frequency range f,10 4 Hz. The highest dielectric constant was 6?2610 4 at f5100 Hz with Zn substitution of x50?8. The improved dielectric properties are believed to be related to the presence of a thin grain boundary barrier layer. The resistivity of the grain boundary decreased largely with Zn substitution as evidenced from the impedance plots.