In the present study, tests were conducted to investigate the behaviour of sulfur capture by calcium based sorbents during oxy-fuel circulating fluidized bed combustion at high oxygen concentrations via a 1MW(th) pilot scale oxy-fuel circulating fluidized bed facility. The effects of operating temperature, overall oxygen concentration, and Ca:S molar ratio were taken into consideration. It was determined that the efficiency of sulfur capture during oxy-fuel combustion was higher than that during air combustion. Meanwhile, with the increase of oxygen concentration, the efficiency of sulfur capture increased. Comparing with the optimum temperature for sulfur capture during air combustion, the optimum temperature during oxy-fuel circulating fluidized bed combustion was about 930 degrees C when the overall oxygen concentration was 50%. In addition, the optimal Ca:S molar ratio was similar to 3.0 during oxy-fuel combustion when various factors were all taken into consideration.