The carbonation/calcination loop of CaO/CaCO3 is an efficient process for CO2 capture. This study investigated the CO2 capture capability of CaO powders derived from oyster shells and reagent-grade CaCO3. The oyster shell-derived CaO powder had an oxide impurity content as high as 9 wt%, and a larger CaO crystal grain size and smaller specific surface area than the CaO derived from the reagent-grade CaCO3. In cyclic CO2 capture tests, the cyclability and CO2 capacity of the oyster shell-derived CaO was significantly improved by inserting an intermediate cooling step between carbonation and calcination. At a carbonation temperature of 740°C, the overall performance of the oyster shell-derived CaO in cyclic carbonation was superior to that of the CaO from the reagent-grade CaCO3. On the basis of X-ray diffraction analysis, it was suggested that the impurities contained in the oyster shell-derived CaO may have constituted transition zone on the CaO crystal grain-boundary to suppress crystal growth in calcination as well as to ease up lattice expansion in CO2 fixation. The intermediate cooling enlarged the transition zone to mitigate lattice dislocations resulting from CO2 fixation and thus, the decay in CO2 capacity.