Pore size distribution and mean pore diameter of calcined lime powder produced by a powder-particle fluidized bed were measured and the effects of reaction temperature, average residence time of gas in the bed, CO₂ and conversion of limestone were investigated. In CO₂- free atmosphere, small pores of less than 100Å were mainly observed in the calcined lime. The mean pore diameter increased with the superficial residence time of gas in the bed, which is defined as the static bed height divided by the superficial gas velocity. When the superficial residence time of gas in the bed was constant, a strong relationship was found between the conversion of limestone and the mean pore diameter of calcined lime. Through this study, it was shown that calcium oxide powders with various pore structures can be easily produced in very short residence time of gas by the powder-particle fluidized bed.