The mechanical properties of ZrO_2-Al_2O_3 ceramic composites have been studied. It is found that there are two suitable ranges in ZrO_2-Al_2O_3 system. For the ceramics with Al_2O_3 matrix, toughening and strengthening of alumina ceramics can be obtained by the addition of ZrO_2 grains. For the ceramics with ZrO_2 matrix, the strength and fracture toughness of ZrO_2-ceramic can be increased by the slight addition of α-Al_2O_3. This is because the crack deflection caused by α-Al_2O_3 cart strengthen the tra...