Thermal stability improvement of mesoporous zirconia is important for its applications as catalyst supports or membrane layers at high temperatures. Yttria-coated zirconia is synthesized by doping a small amount of yttrium nitrate solution in zirconia sol. The sol-gel derived undoped and 2 and 3 mol% yttria-doped zirconia samples after calcination at 450 degrees C are in the metastable phase. Undoped zirconia starts to transform to thermodynamically stable monoclinic phase at 600 degrees C. The yttria-doped zirconia samples remain in the metastable phase after heat treatment in air at 900 degrees C for 30 h. Doping yttria also retards the sintering and grain-growth rates of the sol-gel derived zirconia. Doping with 2 mol% yttria gives the most effective results with respect to retarding the surface area loss of zirconia at high temperature. Yttria with a loading up to 2 mol% is possibly present on the grain surface of zirconia. The improvement of the thermal stability of zirconia by coating yttria is explained in terms of the effects of dopant on the mobility and concentration of the surface defects.