Combustion experiments of two Chinese coals were conducted in a 50 kWth circulating fluidized bed (CFB) combustor in air and O2/CO2 atmospheres, and the effects of atmosphere and O2 staging on combustion characteristics including temperature distribution profiles, combustion efficiency, gaseous pollutant emission and desulfurization efficiency were investigated. Results show that O2 enrichment in the atmosphere elevates the temperature level in the riser and improves the combustion efficiency. Without limestone addition, O2 enrichment in the atmosphere augments SO2 and NO emission and reduces CO emission. The desulfurization efficiency under higher O2 concentration is higher with the same Ca/S molar ratio in O2/CO2 atmosphere. O2 staging provides more operation flexibility for oxy-fuel CFB combustion. As O2 concentration in the primary gas increases, combustion efficiency and desulfurization efficiency are improved. O2 staging can reduce the NO emission to a large extent in O2/CO2 atmosphere.