Cyclic reaction performances and pore structural change of Ca-solid reactants for the CaO/CO₂ chemical heat pump designed for upgrading and storing high-temperature thermal energy were studied. It was observed that solid reactants prepared using CaCO₃ particles have micro-order pore structure among particles and nano-order pore structure in the particles. With the proposed model, in which the pore structure change during reactions is considered, the numerical analysis suggested that the decarbonation rate was determined by nano-order pore structure change. In experiments of cyclic reaction at 923 K, pore volume change corresponding to the greater part of the total volume change between carbonation and decarbonation was observed in the range from 10 to 30 nm of pore radius.