Based on photolithography and microcontact techniques, the current–voltage characteristics of grain boundaries in a ZnO varistor under the 8/20 s pulse current stress were investigated. The electrical properties of 100 grain boundaries of the degraded ZnO varistor sample were directly measured and compared with those of the undegraded sample. The statistical results intuitively show that the nonlinear coefficients decrease and the leakage currents increase among most of the grain boundaries due to the pulse current. The mean breakdown voltage of a single grain boundary is around 3.2 V and does not show obvious variation between those two samples. However, several grain boundaries became conductive after being subjected to pulse current stress.