A rotating packed bed (RPB) is one of the typical process intensification equipment. Its zone between the packing and the casing (cavity zone) is an important mass transfer zone. However, the liquid flow pattern, droplet diameter and size distribution in the cavity zone of a RPB, which is essential for the mass transfer modeling and performance, are still unclear. In this work, the liquid flow in the cavity zone of a RPB was studied under different experimental conditions using photographs taken by a high- speed camera. Two typical liquid flow patterns and liquid disintegrating modes were observed. A flow pattern transition criterion for the transition of ligament flow to droplet flow was presented. Effects of rotational speed, liquid initial velocity, outer packing radius, liquid viscosity, and liquid surface tension on the average droplet diameter and droplet size distribution were investigated. Furthermore, a correlation of the average droplet diameter was proposed. The predicted values of the average droplet diameter were found to be in agreement with the experimental values with deviations generally within +/- 15%.