Abstract: The minimal oxygen content in molten steel under the condition of[Al]-[O]equilibrium at atmospheric pressure condition as well as that under the condition of[C]-[O] equilibrium at various pressures was determined by thermodynamic analysis.Using aluminum as deoxidizer,the mass fraction of[Al]and[O] in molten steel shows U-type relationship at a fixed temperature.The mass fraction of[O]in molten steel cannot be effectively decreased when the mass fraction of[Al] in molten steel is greater or smaller than the lowest value of[Al]. Increasing temperature can reduce the minimal theoretics oxygen content in molten steel equilibrated with aluminum. It is well-known that decreasing the pressure or increasing the vacuum degree can largely improve the deoxidization ability of carbon. However,improving the mass fraction of[C]in molten steel is also an alternative method to decrease the mass fraction of[O]through[C]-[O]equilibrium at a fixed value of pressure or vacuum degree. The deoxidizing ability of carbon through[C]-[O]equilibrium will be greater than that of aluminum by[Al]-[O]equilibrium at 1873 K under conditions of the mass fraction of[C]greater than 0.0015% as well as the pressure is 100 Pa. Actual data is to used to test the minimum value theoretical formula of[Al]and[C]deoxidation which the results are well in agreement with that of the practical results.