Precursor powder of CaCu3Ti4O12 is prepared by a simplified coprecipitation process, in which an optimum reaction condition with ammonium acetate is used as buffer solution and pH=3.0 is proved by X-ray diffraction and scanning electron microscope. The CaCu3Ti4O12 ceramics samples are prepared by sintering the calcined powder at different temperatures (1040℃-1100℃). It is found that higher sintering temperature of CaCu3Ti4O12 ceramics will lead to lager grain size, higher dielectric constant and lower dielectric loss. The dielectric loss of CaCu3Ti4O12 ceramics is suggested to be due to DC electric conductivity, low-frequency relaxation loss and high-frequency relaxation loss. The low-frequency and high-frequency relaxations are related to grain boundary and oxygen vacancy defects respectively.