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High-temperature internal friction of unidireetionally solidified Al_2O_3/YAG eutectics and sintered Al_2O_3/YAG composites were examined up to 1400℃ in an ambient atmosphere, in comparison with Al_2O_3 single- crystal. The internal friction for all samples slightly increases with increasing temperature at above 1000℃. The internal friction of the sintered specimen at 1400℃ 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 Al_2O_3 single-crystal. The difference in internal friction values among these samples strongly depends on the Al_2O_3/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 110 kJ/mol for the sintered composite, the unidirectionally solidified Al_2O_3/YAG eutectic composite, and the Al_2O_3 single-crystal, respectively. The internal friction values at each temperature and/or the apparent activation energy for unidireetionally solidified Al_2O_3/YAG eutectic composite and Al_2O_3 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.