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 are analyzed on .the grounds of microstructure and lattice defects. The conductivity of the crystal grain boundary phases is calculated with the effective-medium theory and found to be three times higher than that of the matrix, and an idea of further enhancing the ionic conductivity of ZrO2 by modifying grain boundaries is thus proposed. The effect of the ZrO2 grain boundaries on the emf of the oxygen sensor is discussed with simplified models.