High-temperature internal friction of unidirectionally solidified Al₂O₃/YAG eutectics and sintered Al₂O₃/YAG composites were examined up to 1400°C in an ambient atmosphere, in comparison with Al₂O₃ single-crystal. The internal friction for all samples slightly increases with increasing temperature at above 1000°C. The internal friction of the sintered spcimen at 1400°C was about seven times higher than that of the unidirectionally solidified one. The internal friction of unidirectionally solidified composite was slightly higher than that of Al₂O₃ single-crystal. The difference in internal friction values among these samples strongly depends on the Al₂O₃/YAG interfacial area per unit volume and may also be related to the characteristic of the grain boundary. The apparent activation energy calculated from the slope of Arrhenius plots was 180, 110, and 110kJ/mol for the sintered composite, the unidirectionally solidified Al₂O₃/YAG eutectic composite, and the Al₂O₃ single-crystal, respectively. The internal friction values at each temperature and/or the apparent activation energy for unidirectionally solidified Al₂O₃/YAG eutectic composite and Al₂O₃ single-crystal are almost the same, which implies composite behaving as a material having no-interface. The shear modulus for all the tested samples gradually decreased with increasing temperature.