More knowledge has to be acquired on the gas-liquid mass transfer characteristics with microbubble aeration before an efficient bioreactor is designed. Experiments about this subject have been conducted in a stirred reactor. Some valuable conclusions were obtained: the shaft power consumption is little affected by the superficial gas velocity (v(s)) and the power consumption of the microbubble generator is rather high; the dispersion of microbubbles is more uniform compared to sparger aeration; surface aeration leads to a reduced microbubble number and the diameter of the microbubble increases with higher superficial gas velocity; when surface aeration happens, the k(L)a-v(s) curves turn quite abnormally different from that in the case of sparger aeration, and correlation-based calculations fit well with the measurements.