The most important factors influencing the performance of an external loop airlift reactor (ELALR) are local hydrodynamics and bubble size distribution (BSD). In this work, we studied the hydrodynamic characters and BSD in a laboratory scale ELALR. The velocity field of the riser was captured using the particle image velocimetry (PIV) technique and the bubble size distribution (BSD) was determined by digital image analysis (DIA). The results of the PIV measurement show that the flow has an unsteady structure, and the wake oscillation phenomena of the bubble and the vortex shedding are periodically asymmetrical. The gas holdup calculated by DIA correlates with the experimental data when the superficial gas velocity is < 0.02 m center dot s(-1). With an increase in the superficial gas velocity, the probability density function (PDF) curve becomes flatter, changing from the unimodal to the bimodal. For the bimodal PDF curve, the peak of the small bubble is similar to 1.5 mm, while the peak of lager bubble shifts from 4.5 to 5.5 mm with the increase in the superficial gas velocity. This work helps to provide insight into the mixing performance of the ELALR.