A gas-liquid exlemal-loop airlift reactor with a riser 0.47 m in diameter and 2.5 m in height and Iwo externalloop down-comers 0.08 m in diameter and 2.5 m in height were used to investigate the gas-liquid two-phase flow structure. Local phase holdups were measured simultaneously by a microconductivity probe with air as the gas phase and water as the liquid phase over a wide range of operation conditions. Liquid flow velocity measurements were performed using the electrolyte tracer measurement (ETM) technique. The hydrodynamics near the sparger zone, riser disengagement zone (zone 1), junction zone (zone 2), and down-comer disengagement zone (zone 3) were systematically examined using the CFDs at the local scale and at the riser scale, respectively. The simulation results showed that zones 1, 2, and 3 exhibit three different flow regimes, which were the secondary mixed flow regime, the mixed flow regime, and the homogeneous bubble regime, respectively. It was also indicated that turbulent kinetic energy and turbulent kinetic energy dissipation rate were influenced by a gas sparger. These results were necessary to explain these different regimes using computational fluid dynamics (CFD) to provide deeper insight at the local scale for reactor geometry, such as gas sparger, junction and disengagement zones as well as the gas-liquid phase flow microstructure. The simulation results at the local scale were difficult to obtain by experiment. The numerical simulating results of local gas holdup and local gas and liquid velocities agreed well with the experimental data at a low gas flow rate. However, large errors occurred in the simulations at a high gas flow rate, because of poor estimation of the influence of bubble-induced turbulence or the higher density of the tracer and the poor mesh refinement. The flow structure and turbulence parameters of the phases presented here were useful for designing gasliquid external-loop airlift reactors.