The direct experimental data for droplet breakage frequency and daughter droplet size distribution is urgently required for the application of the population balance model. To meet this requirement, a method to directly measure the droplet breakage in the pump-mixer was developed using a high speed online camera. Two typical breakup patterns, tensile breakup and revolving breakup were observed and the number of the breakup fragments was determined. The multiple breakage was treated as a series of binary breakups. In order to precisely describe the daughter size distribution, these binary breakups were divided into three different types, i.e. the original tensile breakup, intermediate tensile breakup, and revolving breakup. The influences of the rotating speed and interfacial tension on the droplet breakup frequency and daughter droplet size distribution were then quantitatively investigated. Empirical correlations were proposed and good agreement was found between the prediction results and the experimental data. (C) 2018 Elsevier Ltd. All rights reserved.