To study mechanism of the slag coating on lining of LD converters, uncoated and coated high carbon MgO bricks have been recovered and made into polished and thin sections for optical and electron optics investigations. Slag coating practice is first to drain about two-thirds of the slag after tapping, and add dolomite then rock the furnace for about 3 minutes. Formation of the dense MgO layer was found to be the major mechanism of slag attack resistance in the slag coat on high carbon MgO brick. Dissolution of dolomite made coats of high melting temperatures and protected the bricks, so that the use of burnt dolomite is preferred. In addition, gaseous reaction products of MgO+C resulted in reduction of iron oxide in the slag with MgO enriched, which decreased the corrosion potential of the slag. Fused MgO grains showed better resistance to slag attack than sintered MgO grains, thus remaining protruded which served as an anchor for the slag coating. The use of slag splashing technologies that, provide selective distribution of the slag coating on the highly worn region in the furnace lining such as trunnion areas will increase the BOF lining life.