Abstract: The reason of Si volatilization during the BOF steelmaking process using limestone for slagging was analyzed. After thermodynamic calculation,the temperature and atmosphere conditions that SiO exists stably near the surface of molten iron with high carbon concentration and low temperature were obtained. Results show that the SiO generation reaction only takes place in the "fire zone" in oxidation reactions between some elements in molten iron and CO₂. At the temperature of 1400 to 2300 K,the value of p_CO/p_CO2 that makes SiO exist stably decreases gradually with the increase of temperature,which equals to 10^-25~10^-13 magnitude of the partial pressure of oxygen. Under the condition of adding limestone in 2 min,the partial pressure of SiO is about 10^-2 magnitude when[Si]volatilizes,which is consistent with the practical production.