This paper discussed hydrothermal reactivity of Ca(OH)2 and classified bituminous fly ashes which were collected at an electrostatic precipitator in coal fired power plant at 181$^{circ}C$. The major products of hydrothermal reaction were tobermorite and hydrogrossularite because bituminous fly ashes contained Al2O3 content greater than 20 wt%. As increasing amount of Al2O3 content greater than 20 wt%. As increasing amount of Al2O3 in glass phases, formation of hydrogrossularite increased. Formation rate of crystalline tobermorite increased with content of finer particles, higher glass content and more Al2O3 in glass phases. There was a positive correlation between residue on 45${mu}{ extrm}{m}$ sieve and hydrothermal reactivity of fly ash up to 3 hours. The hydrothermal reactivity of fly ash at 181$^{circ}C$ for 12 hours was more affected by fineness than by glass content of fly ash.