The progresses of the relative density, average grain size (GS), and maximum pore size entering into the final sintering stage are investigated in 3mol% yttria-stabilized tetragonal zirconia polycrystals (3Y-TZP) compacts in the range of 87%similar to 99% theoretical density. It is found that during conventional pressureless sintering, the pores in the compact enlarged dramatically, which appears to be the major obstacle for preparing fully dense nanoceramics. Transparent 3Y-TZP nanoceramics with an average GS of 87nm were prepared by exploiting microstructural refinement on nanoscale. The yields a maximal optical transmittance of 67% and shows no noticeable low-temperature degradation after 100h aging at 134 degrees C under a hydrothermal pressure of two bars.