The effect of microstructure on the ionic conductivity of Zr0.84Ca0.16O1.84ceramic specimens was evaluated. At low temperatures (<800°C), those specimens of smaller grain size exhibited a higher electrical conductivity. These data suggest high grain boundary conductivity owing to a higher diffusion rate for the oxygen ion along the grain boundary. Activation energy vs logarithm of average grain diameter data gave a linear plot; thus, an estimate of the activation energy for grain boundary conduction could be obtained by extrapolation. This value was found to be about 0.96 eV, whereas the activation energy for the bulk specimen lies in the range 1.24 to 1.30 eV. At temperatures of 1000°C or higher, the difference between grain boundary and bulk conductance is insignificant.