Abstract: The minimal oxygen content in molten steel under the condition ofAl-Oequilibrium at atmospheric pressure condition as well as that under the condition ofC-O equilibrium at various pressures was determined by thermodynamic analysis.Using aluminum as deoxidizer,the mass fraction ofAlandO in molten steel shows U-type relationship at a fixed temperature.The mass fraction ofOin molten steel cannot be effectively decreased when the mass fraction ofAl in molten steel is greater or smaller than the lowest value ofAl. 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 ofCin molten steel is also an alternative method to decrease the mass fraction ofOthroughC-Oequilibrium at a fixed value of pressure or vacuum degree. The deoxidizing ability of carbon throughC-Oequilibrium will be greater than that of aluminum byAl-Oequilibrium at 1873 K under conditions of the mass fraction ofCgreater than 0.0015% as well as the pressure is 100 Pa. Actual data is to used to test the minimum value theoretical formula ofAlandCdeoxidation which the results are well in agreement with that of the practical results.