The statistical AC ageing characteristics of single grain boundaries of ZnO varistor were investigated in this paper using the microcontact measurement method. ZnO varistor sample deposited with microelectrodes was degraded under the ageing stress of 0.85 V-1mA at 135 degrees C. The electrical properties of 100 grain boundaries were measured after ageing with different time. The results indicate that the distribution of nonlinear coefficients concentrates to the lower region and the leakage current gradually increases in the ageing process. Meanwhile, more grain boundaries with lower breakdown voltage emerge as the ageing time increases, which means that the nonlinearities of these grain boundaries become weak. (C) 2010 Elsevier B.V. All rights reserved.