This study has been investigated to analyze the thermal stability of cesium reacted with fly ash with changing mole ratio of Cs/Al in hydrogen atmosphere. When the $CsNO_3$ and fly ash were reacted at $1000^{circ}C$ in hydrogen atmosphere, cesium $nepheline(CsAlSiO_4)$ Phase began to emerge in addition to $pollucite(CsAlSi_2O_6)$ phase when the cesium loading quantity was greater than $0.32(g-Cs_2O/g-fly; ash)$. Cesium $nepheline(CsAlSiO_4)$ Phase increased with increasing cesium loading quantity. When cesium trapped on a fly ash was exposed to $1200^{circ}C$ in hydrogen atmosphere, no weight loss due to the volatilization was shown until the cesium loading quantity was reached at $0.32(g-Cs_2O/g-fly; ash)$. In the case of the cesium loading quantity of $0.48-0.74(g-Cs_2O/g-fly;ash)$ in hydrogen atmosphere, the weight loss increased with increasing the cesium loading quantity. This is considered to be due to the cesium $nepheline(CsAlSiO_4)$ whose vapor pressure is higher than that of $pollucite(CsAlSi_2O_6)$.