The effect of scale on coefficient of friction in hot rolling was investigated by changing the scale thickness of Si-Mn steel, using the simulation testing machine developed by the authors. The simulation testing machine for the evaluation of the lubrication behavior consisted of a main stand, a substand, a furnace and a tension device. The rolling force and the rolling torque were measured and the coefficients of friction were calculated by changing the scale thickness, the reduction and the emulsion concentration. The scale thicknesses were obtained by the control of the flow rate of Ar gas and the heating time at 800 C in the image furnace. The experiments were carried out at constant rolling conditions of a velocity ratio of 20, a rolling speed of 50 m/min and a furnace temperature of 800 degrees C at the rolling reductions of 0.3, 0.5 and 1.0 mm. The colza oil was used as the base oil. The emulsion concentrations were 0.1 and 3.0 mass%. At an emulsion concentration of 3 mass%, the coefficient of friction at rolling reductions of 0.3 and 0.5 mm remains constant above 60 mu m, whereas below 60 mu m it increases with decreasing scale thickness. The coefficient of friction at a rolling reduction of 1.0 mm remains constant above 110 mu m, whereas below 110 mu m it increases with decreasing scale thickness. At an emulsion concentration of 0.1 mass%, the coefficient of friction at a rolling reduction of 0.3 mm increases with increasing scale thickness above 60 mu m, whereas below 60 mu m it increases with decreasing with scale thickness. The coefficient of friction at a rolling reduction of 0.5 mm increases with increasing scale thickness above 110 mu m, whereas below 110 mu m it increases with decreasing with scale thickness. The coefficient of friction at a rolling reduction of 1.0 mm remains constant above 110 mu m, whereas below 110 mu m it increases with decreasing scale thickness. The increase in the coefficient of friction for the specimens with a thinner layer of scale is due to the formation of white zones in which the scale is removed from the interface between scale and steel. For these specimens, the ratio of the Fe(3)O(4) layer of the scale surface is higher and the black layer is observed on the roll surface after rolling.