Abstract: Fine bubbles will create when the inert gas is introduced to the high rapid steel stream within the shroud nozzle between ladle and tundish. The collision and attachment among the bubbles and fine inclusions will promote the floatation efficiency of inclusions in the tundish. The behaviors of the bubbles, such as the dispersion in shroud, coalescence and floatation in tundish, are studied.The results show that the maximum sizes of the bubbles in the water and steel flow within the shroud in the length of 1.2 m are 0.70-1.44mm and 1.53-3.16mm respectively when the flow rates are 0.006-0.016 m₃/s; the terminal velocities of fine bubbles in the water and molten steel within the tundish are 0.02-0.2 and 0.05-0.6 m/s.