The effects of the unaerated liquid height (0 less than or equal to h less than or equal to 0.20 m), the superficial gas velocity (0.02 less than or equal to U-G less than or equal to 0.18 ms(-1)) and the downcomer-to-riser cross-sectional area ratio (0.11 less than or equal to A(d)/ A(r) less than or equal to 0.53) on the hydrodynamic characteristics, such as the gas holdups in the riser an downcomer, the circulation time, the circulation liquid velocity and the mixing time, in an external-loop airlift reactor were systematically investigated, The unaerated liquid height, the superficial gas velocity and the downcomer-to-riser cross-sectional area ratio were important parameters which strongly affected the hydrodynamic characteristics of the airlift reactor. As the unaerated liquid height increased, the gas holdups in the riser and the downcomer. the circulation time and the mixing time decreased whereas the circulation liquid velocity increased.