The study of polarization and relaxation behavior for transition-metal oxides in alternating electric field is meaningful for the interpretation of their transport mechanism, dielectric and ferroelectric properties. The dielectric permittivity, relaxation and impedance properties above room temperature for CaCu₃Ti₄O₁₂ (CCTO) ceramic were studied by dielectric analyzer in a wide range of temperature and frequency. The fitting results of impedance indicate that the activation energy for high temperature is equal to that of electric conduction of grain boundary, which is different from the dielectric relaxation mechanism below room temperature. Our results suggest that the dielectric relaxation at high temperature is originated from the grain boundary effects of polycrystalline ceramic.