Abstract: By use of the pressure sensitive stress criterion and the method of weight function, the fracture toughness enhancement induced bymixed modeⅠ-Ⅱcracks in phase transformation toughened ceramics is predicted theoretically. The theoretical expressions for the toughening effects of both the stationary and the steady-state growing cracks are given respectively. Results show that there is a negative effect from the stationary cracks and it increaseswith the increase ofKⅠ/KⅡ, and that the toughening effect of the steady- state growing cracks is not only associated with the modulus of elasticity, the width of transformation area and the transformation volume fraction, but also decreases with the increase ofKⅠ/KⅡ.