A method for estimation of the volumetric mass transfer coefficient of an entire aeration tank for optimization of tank design was investigated. The gas holdup, bubble diameter, and volumetric mass transfer coefficient of an aeration tank were measured by experiments and the experimental results were compared with two estimation method. One method used empirical equations, while the other used CFD simulation. The rate of oxygen transfer into the aeration tank was assumed to be the sum of the rates at the bubble-liquid interface and free surface. The empirical equation method showed good agreement with the measured gas holdup by assuming that the aeration tank was divided two zones. On the other hand, the CFD simulation method provided a good estimation of the gas holdup and bubble diameter. The sum of the estimated volumetric mass transfer at the bubble-liquid interface and free surface showed good agreement with the measured volumetric mass transfer coefficient of the entire aeration tank.