Abstract: Mechanical properties and toughening mechanisms of three ceramics, 99.5vol% Al₂O₃ (AD995) ceramic, ZrO₂(15vol%)/Al₂O₃ ceramic and ZrO₂(25vol%)/Al₂O₃ ceramic prepared by hot pressing sintering were investigated by experimental and theoretical research. Based on the micro-mechanics theory of composites, a micromechanical constitution model was developed to describe mechanical properties of ZrO₂/Al₂O₃ considering phase transformation of ZrO₂. The results show that the grain size of Al₂O₃ matrix decreases and densification is improved with addition of ZrO₂. The failure of three ceramic specimens show characteristic of small deformation to brittle failure. The compression stress-strain curves show an approximately linear relationship. The fracture toughness of AD995 ceramic is 5.65 MPa·m^1/2, while ZrO₂(25vol%)/Al₂O₃ is 8.42 MPa·m^1/2, approximately 50% improvement.With toughening phase ZrO₂ content increasing, elastic modulus of ZrO₂/Al₂O₃ decreases and fracture toughness increases, which are consistent with experimental results.