The effect of CoO addition into Y-TZP (Yttria doped Tetragonal Zirconia Polycrystals) was studied on the evolution of its sintering ability, grain size, grain boundary structure and mechanical properties. The doping of a small amount of CoO effectively reduced the sintering temperature. A small amount of CoO up to similar to 0.3 mol% was effective for the suppression of grain growth, but the addition of 1.0 mole % resulted in an enhanced grain growth. The hardness and toughness of the CoO doped TZP were about the same as those of undoped TZP. Furthermore, despite the grain refinement, CoO doped TZP did not exhibit improved mechanical properties. This may be suggesting that CoO dopant had weakened the grain boundary strength.