Cyclic CO2 capture using CaO-based sorbents derived from commercial pure micro-sized CaCO3 and nano-sized CaCO3 was investigated, focusing on the different characteristics of carbonation conversions, carbonation rates, surface areas, pore volumes, morphological changes, and microstrains of two sorbents during high-temperature reactions. The results indicated that the CaO-based sorbent derived from nano-sized CaCO3 (NC-CaO) provided higher carbonation conversions and carbonation rates than the CaO-based sorbent derived from micro-sized CaCO3 (MC-CaO) in the cyclic CO2 capture reactions. Furthermore, NC-CaO retained its fast carbonation rate at the beginning of each cycle for several tens of seconds. In contrast, the carbonation rate of MC-CaO diminished with an increase in the cycle number. Unfortunately, NC-CaO sintered more easily. Its grains, which were composed of numerous spherical nanocrystallites, suffered from dramatic morphological changes during high-temperature reactions. A mechanism of grain boundary migration was employed to explain the sintering of CaO-based sorbent. The smaller crystallites were more susceptible to be merged by the bigger crystallites during high-temperature reactions.