Water model experiments were performed to measure the mixing time and to investigate the effect of nozzle depth in a ladle-refining process. The depth of the submergence nozzle was varied by 6.0, 7.2, and 8.4 cm, which correspond to fractional depths of 0.5, 0.6, and 0.7, respectively. A new technique was proposed in the present study to measure the mixing time using optical image processing. The mixing time for fractional depths of 0.5, 0.6, and 0.7 determined is 26, 19, and 20 sec, respectively. The gas dispersions in the plume zone are distributed asymmetrically. An increase in the nozzle depth, which enhances recirculation speeds, leads to a decrease in mixing time.