Undercooled melt was produced in the alumina-rich portion of the Al2O3-Y2O3 system, heating the metastable eutectic structure to above the metastable eutectic temperature but below the equilibrium eutectic temperature, followed immediately by solidification along the equilibrium path in the undercooled melt. Coupling the melting and solidification enabled a nearly adiabatic transformation from the metastable eutectic structure to the equilibrium eutectic structure. The eutectic structure produced through the solidification was uniform and fine throughout the castings. A fin-type casting with a thickness of 300 mum was successfully produced. This paper proposes a novel casting process using the undercooled melt formation.