Abstract: The formation and transformation of MgO·Al₂O₃ based inclusions in the gas cylinder steel produced by EAF-LF-VD-CC process flow have been studied. The three-dimensional inclusions were observed by SEM. The results show that after LF refining for 30 min, the content of Mg decreases and that of Ca increases in the inclusions, and MgO·Al₂O₃ inclusion disappeares;at the end of LF refining, the content of Mg in the inclusions has little changes, the content of Ca in the inclusions decreased, and no MgO·Al₂O₃ inclusion exists;during VD refining, the content of Mg in the inclusions increases, the content of Ca in the inclusions has little changes, and MgO·Al₂O₃ inclusion appears again;MgO·Al₂O₃ inclusion can be modified during refining, but to avoid forming MgO·Al₂O₃ inclusions again, the content of Ca in the molten steel have to reach a certain value.