When using a fluidized bed for a limestone calciner, calcinimg reaction rate is decided by the process of CO2 diffusion in quicklime produced outside of the CaCO3 unreacted core in the crystal. The CO2 effective diffusion coefficient obtained by bench-scale testing has a relation to limestone' s crystal diameter. So CaCO3 unreacted core model has been developed based upon CO2 diffusion process. Calculated CO2 effective diffusion coefficients decrease with increaseing temperature of fluidized bed and do not depend on species of limestone.