Crystallization behavior during annealing of eutectic glass in the C12A7–CaYAlO 4 system was investigated by SEM, STEM, XRD, and TG–DTA. At 900°C, phase separation of Y-poor and Y-rich phases appeared without crystallization. The phase separation might lead bulk crystallization of C12A7 by subsequent crystallization. At 950°C, crystallization of unknown metastable phase was observed together with C12A7 crystallization. Above 1000°C, crystallization of CaYAlO 4 and C12A7 was observed. Crystallization behavior was schematically explained and bulk crystallization of C12A7 was explained by the phase separation.