Abstract: The decomposition process of phenolic resin which is a binder of spherical fuel elements matrix material for high temperature gas-cooled reactor（HTGR） was studied by means of TG-IR and DSC at the temperature of 20～800 ℃.The thermal expansion characteristics of graphite ball samples were investigated through thermal mechanical analysis（TMA）.The results showed that the phase transition of the phenolic resin occurred at the temperature of 50～70℃ and the decomposition had two continuous exothermic stages.Accordingly,the graphite ball samples were expanded and shrink successively during carbonization.The properties of the matrix material made through four-stages heating process completely satisfied the design requirements and the production efficiency of carbonization was increased by 71%.Studies showed that the heating process established according to the sample’s size varied with carbonization temperature under dynamic conditions is more reasonable and the volume change caused by condensation is the critical factor to heating rate.