Abstract: The sulfur-retention properties and microstructures of Shenmu coal ash are studied. It is found that the self-desulfurization efficiency of Shenmu coal with an initial molar ratio Ca/S of 1.45 dramatically decreases from 63.5% to 6.4%, when the furnace temperature increases from 800 ℃ to 1 200 ℃. The reason is that the active calcium (CaCO3 and CaO) content is 22.4% to give a sulfation product CaSO4 content of 18% in the combustion residue at 800 ℃. It is composed of many small particles with fine pores (specific surface area of 4.6 m2·g－1 and mean pore diameter of 14.2 nm), which form a loose cotton-like structure and provide large surface area for sulfation reaction. But none of the phases of CaCO3, CaO and CaSO4 is found in the coal ash at 1 200 ℃. It is composed of many large nonporous particles with smooth surfaces (specific surface area of 2 m2·g－1 and mean pore diameter of 5.5 nm), due to the high levels of sintering swell and melting.