Cs-Bi and Cs-Sb absorbed graphite samples (Cs-Bi/graphite and Cs-Sb/graphite) were synthesized and their high temperature chemical stabilities were tested up to 1500 °C by TG and analyzed by TEM-EDS for the development of Cs trap material in high temperature gas-cooled reactor (HTGR) fuel particles. It was observed that Cs was stabilized by Sb but not by Bi in the specimens after the TG test. A rapid weight loss from 800 to 1000 °C may be caused by evaporation of Cs (boiling point: 671 °C) was seen in the TG result of both specimens. Precipitated Cs-Sb substance in the graphite matrix were not resolved even after the 1500 °C heating. The chemical composition of the Cs-Sb was specified as Cs3Sb. The experimental results suggest that Sb have potential to be a Cs getter material in graphite matrix. Long term heating test should be performed to confirm adaptability of Sb for Cs trap material in HTGR fuel particles.