CaO carbonation with CO2 is potentially a very important reaction for CO2 removal from exhaust gas produced in power plants and other metallurgical plants and for hydrogen production by promoting water gas shift reaction in fossil fuel gasification. A mathematical model based on the grain model was applied for modeling of this reaction. Diffusion of gaseous phase through the product layer and structural change of the grains were considered in the model. The modeling results show that ignoring the reaction kinetics controlling regime in the early stage of the reaction and replacing it with a regime considering both the reaction kinetics and diffusion can generate good simulation results. The frequency factor of the reaction rate equation and the diffusivity of CO2 through the CaCO3 layer were justified to get the best fit at different temperature range from 400 to 750 ℃ with respect to experimental data in the literature. The mathematical model switches to a pure diffusion controlling regime at final stage of reaction.