The gas-to-liquid volumetric mass transfer behavior has been studied using the oxygen dissolution method in a three-phase circulating fluidized-bed (CFB) reactor 140 mm in inner diameter and 3.0 m in height. The volumetric mass transfer coeffcient k(L)a is obtained from the measured bulk concentrations of the liquid phase by means of the axial dispersion model. The influences of the main operation conditions, such as superficial gas velocity and superficial liquid velocity and solid circulating rate, on the-volumetric mass transfer coefficient are studied systematically.