A ¼-scale water-model experiment is conducted to investigate the trajectories, velocities and size distribution of bubbles in a continuous-casting mold. A high-speed camera and a laser are used to record the transient bubble distribution. The region of interest is delimited to include the motion range of floating bubbles in the upper mold, and divided into 70 subregions of seven rows and 10 columns. Information on the trajectories, velocities and size distributions of bubbles is obtained from image sequences, using the ImageJ software. The effects of water/gas flow rates on the transport and size distribution of bubbles are studied. The results show that the motion range of larger bubbles decreases toward the submerged entry nozzle (SEN) and the floating direction of larger bubbles moves closer to vertical. The increase in water flow rate increases the level transportation distance of bubbles along the jet flow and the intensity of upper circulation flow, which has a complex effect on bubble transportation near the SEN. An increase in the gas flow rate contributes to the ascending flow near the SEN and significantly increases the bubble number in the region of interest for the entire bubble-diameter range, without causing an increase in mean bubble diameter. Display omitted •Transient trajectories and velocities of various bubbles were firstly obtained from water model experiments.•Bubble size distribution at various water and gas flow rates were analyzed in detail.•Average diameter and number of bubbles at different regions were analyzed.