In view of its potential application to high temperature heat energy storage, carbonation of CaO with various CO2 pressures is investigated in the temperature range 873-1173 K by using a thermogravimetric analyzer under atmospheric conditions. For a given temperature, although the reaction takes place for a partial pressure of CO2, P, higher than the equilibrium CO2 pressure of CaCO3, P-e, the reaction rate shows a slight increase with increasing P, On the other hand, for a given P, the rate increases rapidly with increasing temperature from 973 to 1023 K, above which it decreases gradually. By applying a grain model and assuming the presence of a reaction intermediate, a possible reaction rate equation has been developed.