Based on preparing large bulk Al2O3/ZrO2 (4Y) eutectics, SiO2 additive is used for controlling the microstructures, densification and mechanical properties. XRD pattern showed SiO2 additive in the ceramics was in the form of glass phase. SEM images showed that the microstructure morphologies transformed from the cellular eutectics to the rod-shaped colonies with increasing content of SiO2 additive. With increasing content of SiO2 additive, the relative density of the ceramics increased whereas the hardness of the ceramics decreased. As the content of SiO2 additive reached 4%, fracture toughness of the ceramics had the maximum value due to the coupled toughening mechanisms of crack-pinning, crack- bridging and crack-deflection by rod-shaped colonies. As content of SiO2 additive reached 6%, the highest flexural strength of the ceramics was achieved due to high fracture toughness and small-size critical defect.