Drop size distribution (DSD) or mean droplet size (d(32)) and liquid holdup are two key parameters in a liquid-liquid extraction process. Understanding and accurately predicting those parameters are of great importance in the optimal design of extraction columns as well as mixer-settlers. In this paper, the method of built-in endoscopic probe combined with pulse laser was adopted to measure the droplet size in liquid-liquid dispersions with a pump-impeller in a rectangular mixer. The dispersion law of droplets with holdup range 1% to 24% in batch process and larger flow ratio range 1/5 to 5/1 in continuous process was studied. Under the batch operation condition, the DSD abided by log-normal distribution. With the increase of impeller speed or decrease of dispersed phase holdup, the d 32 decreased. In addition, a prediction model of d 32 of kerosene/deionized system was established as d(32)/D= 0.13(1 + 5.9 phi)We(-0.6). Under the continuous operation condition, the general model for droplet size prediction of kerosene/water system was presented as d(32)/D = C-3 (1 + C4 phi)WC-0.6. For the surfactant system and extraction system, the prediction models met a general model as d(32)/D = b phi(n) We(-0.6). (C) 2019 The Chemical Industry and Engineering Society of China, and Chemical Industry Press Co., Ltd. All rights reserved.