Zirconia nanopowders (stabilized by 1.5 and 3 mol% Y2O3) with particle size below 10 nm were compacted by cold isostatic pressing. Pressureless sintering and hot isostatic pressing were applied to obtain dense nanocrystalline ceramics. The influence of the pore size in powder compacts on sintering behaviour was investigated. Green bodies pressed at 1000 MPa had a maximum pore size of 5 nm. These bodies were densified to a relative density of over 99.6% with an average grain size about 85 nm by pressureless sintering at 1100 °C. Indentation techniques were used to evaluate the hardness and fracture toughness of zirconia nanoceramics. The decrease in the yttria content from 3 to 1.5 mol% resulted in the toughness increasing from 5.3 to 11.1 MPa m1/2. The differences in fracture toughness of zirconia ceramics prepared with different yttria contents and by different sintering methods were discussed and their possible causes were proposed.