Abstract: Composite ceramics of cubic or tetrahedral ZrO₂ phase on the microstructure, phase composition,bending strength,fracture toughness as well as ionic resistivity were tested and the mechanisms were discussed.The results showed that small amount of ZrO₂, especially the partial-stabilized t-phase which can transform to m-phase under stress,could effectively improve the properties of single phase β"-Al₂O₃ ceramics.