The reaction kinetics of the CaO/Ca(OH)(2) reversible chemical reaction was studied by using a vacuumed forced convection type experimental device for analysis of the closed chemical heat storage system. The hydration of CaO was studied in the chemical reaction control region by thermogravimetric analysis for a reactant solid particle size of under 53 pm, a reaction temperature of 503-550 K and a water vapor pressure of 1.0-2.3 kPa. The experimental results obtained by thermogravimetric analysis with the vacuumed forced convection type experimental device agreed with our reaction model. The model is based on the following two assumptions. 1. The hydration of CaO is a reversible chemical reaction. Thus, the final reaction conversion is an equilibrium reaction conversion. 2. The equilibrium reaction conversion is determined by reaction temperature and water vapor pressure, and reflects the reaction rate. The reaction order of the hydration are was found to be 1. 0 with respect to the concentration of solid reactant and water vapor pressure. Two different reaction rate constants were found in the respective temperature regions higher or lower than reaction temperature 533 K. Moreover, the CaO/Ca (OH)(2) reaction in the vacuumed closed system takes place over a wider temperature range than in the open system.