Sulfation and calcination characteristics of pure CaO (lime) and limestone under a CO2 gas atmosphere (0-15%) were determined by using both X-ray diffraction (XRD) and thermogravimetric analysis (TGA). The decomposition temperature of limestone in a calcination reaction has been correlated with CO2 partial pressure. The sulfation conversions of pure CaO and limestone are predicted by quantitative XRD analysis, and the sulfation conversion by an XRD method is good according to that measured by TGA. The sulfation reactivity and sulfur capture capacity of limestone increase with increasing temperature and decreasing CO2 partial pressure in the ambient gas stream. The full width at half-maximum (fwhm) of the main peak (CaO) in the XRD spectrum on the sintered lime decreases with increasing sintering temperature and time. The CaO grain size of sintered lime is determined by a qualitative XRD analysis and correlated with the specific surface area of lime and the true density of CaO.