The present study investigated the mechanism of small bubble breakup in an unbaffled vessel stirred by a 4-blade paddle impeller. Bubbles were injected underneath the rotating blades. Numerical simulation and experimental observations were performed to investigate the related phenomena. The Volume of Fluid (VOF) method was utilized to simulate a gas-liquid multiphase flow. The injected bubbles were broken behind the impeller blade, where they were transported due to a decreasing pressure gradient caused by the impeller rotation. At the behind of the blade, bubbles were elongated and fragmented due to the trailing vortices developing from the rear surface of blade. The fragmented bubbles were entrapped at the cores of trailing vortices, and then floated up in the space where the trailing vortices became weak. The size of the broken bubbles was dependent on the impeller rotation rate. (C) 2018 Elsevier Ltd. All rights reserved.