Characteristics of gas holdup and gas-liquid mass transfer were investigated in the riser of a three-phase circulating fluidized-bed bioreactor where the nitrogenous component was removed from the synthetic wastewater. The diameter and height of the riser were 0.102 and 1.0 m, respectively. Anion polymer resin (diameter: 0.4 mm) was used as a fluidized biofilm media upon which the microorganism was adhered. The effects of the gas and liquid velocities and the holdup of the biofilm media on the gas holdup and gas-liquid volumetric mass transfer coefficient were examined. The gas holdup was determined by means of a dual electrical resistivity probe method. The gas-liquid mass transfer coefficient was recovered from the concentration profile of dissolved oxygen in the axial direction of the riser by adopting the axial dispersion model. We found that the gas holdup increased with increasing gas velocity, but decreased slightly with increasing the liquid velocity or the holdup of the biofilm media. The mass transfer coefficient increased with increasing the gas velocity or the holdup of the biofilm media, but did not change considerably with respect to the liquid velocity in the riser. The gas holdup and volumetric gas-liquid mass transfer coefficient correlated well in terms of the operating variables.