High Carbon(15∼30%) MgO bricks containing aluminum and silicon in addition to carbon were studied to understand the mechanism of their slag resistance. Samples chosen from the hot spots of electric arc furnace walls were sliced into thin pieces and polished. Polished surfaces were investigated using polarizing microscope, electron probe microanalyzer and X-ray diffractometer. As is the case of low carbon MgO brick, the formation of dense MgO layers were verified both in straight and Al and Si containing high carbon MgO bricks. In the Al and Si containing bricks, Si contributes to the densification of MgO layers by the oxidation to SiO₂followed by the formation of forsterite. It is also speculated that aluminum addition resulted in the formation of the dense structure of the brick by forming spinel with magnesium oxide.