Al2O3/ZrO2(Y2O3) directionally solidified eutectic ceramics (DSECs) with different Y2O3 addition were prepared. Three polymorphs of ZrO2: m-ZrO2, t-ZrO2 and c-ZrO2 appeared and disappeared in order with the increase of Y2O3 addition, the effects of Y2O3 addition amount on the mass fraction of ZrO2 polymorphs were quantitatively analyzed. The gradual decrease of area fraction and average size of colony was attributed to the increase of constitutional undercooling region, the variation of interphase spacing in the colonies was explained via selection mechanism of dendrite tip. The maximum hardness and fracture toughness were 18.35 GPa and 8.55 MPa m1/2 obtained at 9 mol% and 3 mol% Y2O3 addition, respectively. The hardness was determined by the mass fraction of m-ZrO2 phase, the area fraction of intercolony and the residual compressive stress, and the toughening mechanisms were composed of microcrack toughening, t-m transformation toughening and residual stress toughening.