In order to investigate the influence of Al2O3 on the FeOx-SiO2 phase diagram, experiments of phase equilibrium were investigated with 0 to 9 mass-% Al2O3, in oxygen potential of log pO2 = - 8.5 to -7 at 1523 and 1573 K. As a result, the liquidus line on SiO2 saturation moves to the lower value side of Fe/SiO2 with increasing Al2O3 concentration in the molten slag. In addition, it was found that the liquidus line of FeOx moves to the lower value side of Fe/SiO2 with the increase of the Al2O3 concentration in the molten slag. It was also found that the uniform melt range spreads with the rise in temperature. In the log pO2 = - 8 and 1573 K, 65 mass-% FeOx and 35 mass-%SiO2 slag which is a general copper smelting slag of Fe/SiO2 = 1.4 shows in a uniform melt at 0 mass-% Al2O3. When the Al2O3 concentration increased over 8 mass-%, Spinel was precipitated in the slag. In the case of the slag of Fe/SiO2 = 1.4, it is necessary to reduce the concentration of Al2O3 in the slag to lower than 8 mass-%. When the oxygen partial pressure increases from log pO2 = - 8 to - 7, the liquidus on the SiO2 side moves to the high Fe/SiO2 side, and the liquidus on the FeOx moves to the lower Fe/SiO2 side, and the homogeneous liquid phase was narrow. In the case of a slag of log pO2 = - 7 and Fe/SiO2 = 1.4, Spinel precipitates at about 5 mass-% Al2O3. An increase in oxygen partial pressure promotes the formation of spinel at lower Al2O3 concentrations.