Abstract: Effects of the chemical compositions and phases of the slags on desulphurization of molten iron and semi-steel were studied.Sulfur content in the desulfurized slag is 4~5 times higher than those of BF slag and vanadium-extracting slag.Sulfur exists in the form of CaS in the desulfurization slag,no MgS has been found.The average content of CaO in the semi-steel desulfurization slag is 15% less than that in the molten iron desulfurization slag,but FeO content is 9% more than that of later slag.The low melting point phases are few in the slag aftrer molten iron desulfurizationand mainly are mCaO·nAl₂O₃(<1 400℃).But for semi-steel desulfurization the low melting point phases has much higher content in the slag,mainly are FeO(1 369℃).Increasing desulfurizer injection amount can improve the sulfur control capacity of desulfurization slag,reducing resulfurization.BF slag as the top slag in the molten iron desulfurization process is 0.01~0.02 t/t,and the same as for vanadium-extracting slag.