In this study, the isothermal section of a Cu2O-Al2O3-SiO2 pseudo-ternary phase diagram at 1150 degrees C was analyzed by means of a scanning electronic microscope and powder X-ray diffraction of the quenched samples qualitatively, and the compositions of the tie-points of the tie-planes as well as their regions were determined by in situ high-temperature quantitative X-ray diffraction analysis and energy-dispersive X-ray spectroscopy. Then, the isothermal section of the Cu2O-Al2O3-SiO2 pseudo-ternary phase diagram at 1150 degrees C was constructed; it was found that the isothermal section is composed of two single liquid-phase regions, five two-phase regions, and six three-phase regions.