Ultrafine eutectic-like microstructures of various rare earth (RE) oxide-Al2O3 systems were formed by use of amorphous phases. This new method uses a low migration rate in the amorphous phases. Mixtures of RE oxide (RE: Yb, Dy, Er, Ho, Gd, Sm, Eu) and Al2O3 powders with the eutectic compositions were melted and quenched rapidly to form the amorphous phases. A heat treatment of the amorphous phases of various eutectic systems at 1000 and 1300 °C, for 30 min, formed RE aluminum garnet (RE3Al5O12)/Al2O3 phases or RE aluminum perovskite (REAlO3)/Al2O3 phases. Scanning electron microscopy observation of these materials heat-treated at 1300 °C showed eutectic-like microstructures, in which crystals of eutectic component were entangled with each other. Furthermore, the microstructures were much finer than those of materials generally prepared from eutectic melts. In this study, it was confirmed that this method is useful for the formation of ultrafine eutectic-like microstructures for many eutectic systems.