Desulfurization efficiency in the powder-particle fluidized bed desulfurizer is investigated in three Ca based sorbent powders limestone, dolomite and scallop shell. The effects of reaction temperature, diameter of the fine sorbent, bed height of medium particles, gas velocity, CO2 concentration of fluidizing gas, and molar ratio of carbonates to inlet SO2 (Ca/S or (Ca + Mg)/S) on desulfurization efficiency are investigated. The specific surface area and pore size distribution of these sorbents before and after calcination or desulfurization are also measured. The desulfurization efficiency oi each sorbent is affected by each operating condition, except for that of scallop shell, being hardly affected by CO2 concentration of the fluidizing gas. Scallop shell powder exhibits the highest desulfurization efficiency. The calcined pore size of scallop shell has a peak pore diameter of about 400 Angstrom, whereas the peaks for the calcined limestone and dolomite are around 80 Angstrom. From this investigation it is suggested that the large size of pores in the calcined sorbent relates to high desulfurization activity.