The gas-liquid mass transfer behavior is studied in a three-phase circulating fluidized bed reactor of 140 mm i.d. and 3.0 m height. Using the oxygen dissolution method, the volumetric mass transfer coefficient k(L)a is obtained from the measured bulk concentration of the liquid phase by fitting to the axial dispersion model (ADM). The gas holdup and the distribution of bubble size in the bed are measured by a fiber optical probe system, then the gas-liquid interfacial area a and the mass transfer coefficient k(L) are calculated. The influences of the main operating conditions, including superficial gas velocity, superficial liquid velocity and solid circulating rate, are studied systematically.