Abstract: In order to explain the influence of content of MnO in slagon the content of silicon in the pig iron, the behaviour of the reaction﹝Si﹞ + 2(MnO)=2﹝Mr﹞ + SiO₂ to make the silicon out of metal and the limiting factors of this reaction are pointed out, based on the variety regularity in the practical data of blast furnace, the data of laboratorial research and of the reference of our country and other countries.