Phase structures, morphology and dielectric properties of prepared CaCu3Ti4O12 ceramics were investigated to better understand the origin of giant dielectric constant in CCTO system. It was found that the samples' dielectric properties obviously vary with sintering conditions and the corresponding dielectric constant can reach more than 5 x 10(4) by sintering at 1100 degrees C for 8(similar to)10 h. XPS results revealed that the ratios of Cu2+/Cu+ and Ti4+/Ti3+ on the surface are apparently higher than that in the interiors, showing CuO segregation and oxidation process on the surface of CCTO ceramics during sintering in air. Therefore, it is significant for CCTO to achieve giant permittivity by properly controlling sintering conditions. According to defect chemistry, the mechanisms of defect formation and recombination are proposed to explain the influence of CuO loss on electrical properties of CCTO-base materials.