On the basis of experiments, the ZrO2 grain boundaries are classified into three types : the "clean" grain boundaries, the grain boundaries with amorphous phases and the grain boundaries with crystal phases. The structure and property of the three types of grain boundaries were analyzed from the aspects of microstructure and lattice defects, and the conductivity of the crystal grain boundary phases was calculated with the effective-medium theory to be as three times higher as that of the matrix, an idea of further enhancing the ionic conduction of ZrO2 by modifying grain boundaries was thus proposed. The effect of the ZrO2 grain boundaries on the EMF of the oxygen sensor was discussed with simplified models.