A custom-built experimental system that can measure sample weight changes at high temperatures is used to study the effect of operating parameters (sintering time, atmosphere, temperature, particle size, etc.) on the sintering of CaO under Ca-looping process conditions. The pore structures of the sintered CaO samples are measured by N-2 absorption/desorption method. The results show that both the rate of carbonation reaction and the final carbonation ratio of the CaO decrease as the sintering duration is increased, especially during the chemical reaction controlled stage. The carbonation of nascent CaO is degraded if steam was added in the calcination process. The sintering of CaO is accelerated by higher temperatures and that will lead to the lower carbonation conversion ratios for the CaO. The highest carbonation ratio is not obtained for the smallest particles (7597 mu m) but for the middle sized particles (150250 mu m) for all the CaO samples for any sintering durations. The biggest CaO particles (355450 mu m) have the lowest carbonation conversion ratio. The surface area and the pore volume of the CaO consistently decrease by the prolonged sintering time. It is interesting to note that for the nascent CaO steam presence will cause the surface area and the pore volume to decrease. But the reverse is true for CaO samples sintered for 6 and 12 min postcalcination. For all CaO samples with different sintering times, steam will change the pore size distribution for pores larger than 200 angstrom. Finally, similar results are obtained for another two different limestones.