The compositions of the grains and the grain boundaries in CaCu3Ti4O12 (CCTO) have been characterized by a high-resolution field emission transmission electron microscope equipped with a high-angle annular-dark-field detector and energy-dispersive X-ray spectroscopy (EDS). The excess ions and their clusters at grain boundaries led to the observed strain contrast. These ions are further clarified as Cu ions by Z-contrast (Z: atomic number) image and EDS. The quantitative EDS analyses further indicate that the grain boundaries of CCTO are Cu rich and the grains are Cu deficient. The Ca- or Ti-rich amorphous phases were observed at corners or edges of the grains of CCTO.