From the viewpoints of energy and environmental problems in the world, the development of effective energy utilization technologies such as chemical heat storage/pump and their practical uses are required. In this paper, the hydration reaction rates of various size CaO particles for calcium oxide/water chemical heat storage/pump were studied using thermogravimetric analysis based on the grain model. Firstly, we calculate a reaction rate equation using the coefficient eta(r) for the particle size effects as model I, which is based on our previous method for calcium sulfate considering the effects of the particle size. Furthermore, the catalyst effectiveness factor taking into account the mass transfer resistance within the porous particles, the particle expansion and the pore size changes along with the hydration reaction is proposed and introduced into a new reaction model II, because the shapes of the calculated data in model I and the experimental data are different. As a result, it is demonstrated that the intraparticle mass transfer resistance increases as the particle size increases. Further, the proposed applied grain model I enables evaluation of this experiment roughly. Furthermore, the applied grain catalyst model II shows that the calculated conversion lines are consistent with the experimental data for 106-1,000 mu m phi particles considering the pressure drop inside the particles by the introduction of the catalyst effectiveness factor. In addition, the expansion ratio of the CaO particles is lower than that of the grains in the reaction model II.