Microstructure of the CaO-TiO2 mixture ground under dry atmospheric condition using a planetary ball mill was investigated by transmission electron microscopy-energy dispersive X-ray spectroscopy (TEM-EDX) and high resolution transmission electron microscopy (HR-TEM) with selected area electron diffraction (SAED) analysis. According to the TEM observation, the mixtures ground for about 2 h or more consist of many fine particles of about 20 nm. The CaTiO3 microcrystals are formed in the mixture ground for about 2 h or more. One of the lattice planes, {121} is clearly observed in the HR-TEM micrograph of the 5-h ground mixture, while the lattice image is partly detected in the mixture ground for about 2 h or less. The grains grow in size in the prolonged grinding, and their sizes reach about 20 nm at 5 h grinding by the TEM observation. The size of microcrystals becomes larger with increasing grinding time.