The corrosion test for magnesia-carbon refractory, MgO-C, with dense layer has been carried out by a rotating cylinder method at 1673 K. The specimen, which was a cylinder of 25 mm diameter and 25mm height, was rotated in molten slag at 50 rpm. The chemical composition of slag was CaO-SiO2-FeO-MgO-CaF2. The corrosion rate was determined by the variation of radius of the cylindrical specimen against corrosion time. The decrease of radius was obtained by a calculation of dissolved MgO into the slag. In addition, the diameter of the specimen was measured with a slide calipers after the corrosion test. The cylindrical specimen was heated to form a dense layer before the corrosion test. The corrosion rate of the MgO-C refractory with dense layer was slower than that of the MgO-C refractory without dense layer. The oxidized layer, where the dense layer formed, dissolved at much the same rate as the MgO brick. The dense layer probably obstructed contact between slag and carbon in the matrix, and then oxidation of C by slag did not take place. Formation of the dense layer was effective to inhibit corrosion of MgO-C refractory by molten slag.