Al2O3/ZrO2(Y2O3) pseudo-binary eutectic melt may solidify in the dendritic form at the low G/v ratio (thermal gradient G divided by growth rate v) associated with the Y2O3 microsegregation along the monovariant line. Especially, in the system with high Y2O3 content (>= 4.5 mol%), an Al2O3/ZrO2/YAG ternary eutectic in situ microstructure growing with a "plane front" will appear at the boundaries of dendritic structures. The volume fraction of the ternary eutectic increases with the increase in Y2O3 content. It was found that the amount of ternary eutectic can be well predicted by the Scheil equation (or nonequilibrium lever rule).