Ultrafine-grained zirconia ceramics doped with yttria in different concentrations from 2 to 12 mol% were prepared by chemical precipitation method with an average grain size of 10 nm. The ac ionic conductivity studies revealed that 3 mol% yttria stabilized zirconia (3YSZ) in tetragonal phase shows maximum conductivity both in grains and grain boundaries for a range of temperatures. In the coarser grained zirconia systems, with different yttria concentration, the conductivity maximum is usually observed in the cubic phase field with 8 mol% Y2O3. The shift of the maximum conductivity with the yttria content towards the lower concentration (3 mol%) is attributed to the grain size effect. The Arrhenius plot of the total de conductivity is found to follow that of the grain boundaries rather than the grains, showing that the total conductivity is mainly influenced by the grain boundaries in the ultrafine-grained materials.