A bubbling fluidized bed reactor was used to study CO2 capture from flue gas by using a potassium-based solid sorbent, sorbKX35 which was manufactured by the Korea Electric Power Research Institute. A dry sorbent, sorbKX35, consists of K2CO3 for absorption and supporters for mechanical strength. To increase initial CO2 removal, some amount of H2O was absorbed in the sorbent before injecting simulated flue gas. It was possible to achieve 100% CO2 removal for more than 10 minutes at 60 oC and a residence time of 2 s with H2O pretreatment. When H2O pretreatment time was long enough to convert K2CO3 of sorbKX35 into K2CO3·1.5H2O, CO2 removal was excellent. The results obtained in this study can be used as basic data for designing and operating a large scale CO2 capture process with two fluidized bed reactors.