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YAG-alumina composites with Al2O3-rich and YAG-rich compositions were prepared by annealing the hot-pressed composites at the eutectic temperature (1820°C) for 1 h followed by cooling down at 10 or 100°C/min in argon atmosphere. Microstructural observations of the solidified composites identified that metastable phase δ-Al2O3 precipitated in the Al2O3-rich specimens cooled down at both the cooling rates. For the solidified YAG-rich specimens, however, equilibrium phase α-Al2O3 could not be reprecipitated, moreover, metastable YAlO3 phase was formed at cooling rate of 100°C/min. Formation of the metastable phases in the solidified composites was attributed to the decreased melting temperature caused by the divorced eutectic precipitation and the metastable decomposition of YAG component in YAG-alumina eutectic melt during rapid solidification.