The absorbent for SO2 from the flue gas of a coal-fired electric power station was prepared from calcium oxide, calcium sulfate, and coal fly ash and examined for the relation between the desulfurization activity and the structure. The activity is closely related to the progress of the hydration reaction taking place during preparation procedures. The activity increased with the hydration time and reached a maximum activity in 12 h. The hydration resulted in the formations of ettringite and calcium silicate. By elevation of the temperature for drying the hydration products, the activity markedly increased up to 400-degrees-C, which was caused by the removal of water covering the calcium component in the ettringite. For the efficient removal of SO2, the existence of NO in the flue gas is required. NO(x) plays a catalytic role for oxidation of SO2 to SO3 which reacts with CaO to form CaSO4 as a final product.