In this paper, the hydration kinetic process of free CaO in system containing high calcium fly ashH 2O has been studied under different temperatures. The degree of hydration reaction at time t was determined by means of the volume expansion value L of hardened Portland cementhigh calcium fly ash pastes. In the present experimental research it was found that the hydration is firstorder reaction whose rate is directly proportional to the degree of reaction. For the hydration reaction the plot of ln L against t is a straight line and the intercept gives initial volume expansion L 0 at t=0 while the rate constant k can be calculated from the slope. It was also found that the rate constant k increases with the increase of the temperature and the quantitative relationship between them followed Arrhenius' law and the plot of ln k against T -1 is linear for the reaction. Both frequency factor A and activation energy E a, which are known as Arrhenius parameters, were calculated. The experiment results also show that the hydration reaction rate of free CaO increases while the volume expansion decreases in the pastes through grinding high calcium fly ash. Some admixtures have influence on the kinetic parameters of the reaction.