In this research, the eggshell was thermally treated at 700°C for 1-3 hrs and 800°C for 1- 4 hrs. After the treatment, thermally treated eggshell was characterized via X-ray diffraction spectrometer, particle size analyzer and thermogravimetric analyzer. From the XRD patterns, it revealed that calcium carbonate was obtained when chicken eggshell was treated at 700°C. The content of calcium carbonate, however, decreased with increasing heating times and temperature. After the eggshell was heated at 800°C, calcium hydroxide was mainly obtained and eventually when the treatment time was 4 hrs, instead of calcium carbonate, calcium hydroxide and calcium oxide were obtained. The size of the treated eggshell was about 0.06 – 878.7 µm depending on the treatment times and temperatures. The size distribution of 800°C-treated eggshell was narrower than that of the 700°C-treated eggshell.