CaO sorbent is one of the most promising sorbents for CO2 capture. However, the reactivity of CaO sorbent decreased rapidly with carbonation-calcination cycles. Steam activation is a feasible approach to improve the sorbent reactivity. In this study, the effects of steam on CaCO3 calcination (CaO regeneration), CaO carbonation and CaO hydration were both investigated. Compared with pure CO2 calcination atmosphere, introducing steam into calcination atmosphere enhanced CaCO3 decomposition rate, which was because that (1) the partial pressure of CO2 decreased; (2) the absorption of H2O by active site CaO* weakened the binding ability between CO2 and CaO*; (3) the amount of heat-transfer between steam and CaCO3 was higher. Lower decomposition temperature in steam/CO2 calcination atmosphere resulted in lower sorbent sintering and higher sorbent reactivity. Besides, the carbonation reactivity of CaO sorbent was over doubled when steam was introduced into carbonation atmosphere, which was due to the formation of OH-. In CaO hydration reaction, sorbent particle pore structure was also developed by hydration treatment. The hydrated Ca(OH)(2) sorbent reactivity as well as cyclic reactivity of CaO sorbent was therefore enhanced. (C) 2016 Elsevier B.V. All rights reserved.