The effects of the addition of Co3O4 to yttria-stabilized zirconia (8YSZ) on fracture toughness and microstructure was investigated using micro-indentation and scanning electron microscopy (SEM). Undoped and 1-15 wt.% Co3O4-doped 8YSZ powders were prepared using a colloidal process and then pelletized under a pressure of 200 MPa. The 8YSZ specimens were sintered at 1400 degrees C for 1 h. SEM results showed that Co3O4 precipitated as a secondary phase among the primary 8YSZ grains. Furthermore, because of the presence of Co3O4 secondary phases at the grain boundaries of 8YSZ, the grain sizes of the 8YSZ samples increased with the addition of 1-15 wt.% Co3O4. The fracture toughness values of the undoped and 1 wt.% Co3O4-doped 8YSZ were determined to be 1.64 and 2.86 MPa m(1/2), respectively, indicating the fracture toughness of 8YSZ increased in 1 wt.% Co3O4 addition. The fracture toughness of 8YSZ increased upon the addition of 1 wt.% Co3O4.