The degradation of ZnO-based multilayer ceramic varistors (MLCV) caused by electro-static discharge (ESD) and its mechanism on Schottky barriers formed at grain boundaries were examined. ESD is an extremely fast pulse which rise time is less than 1 n sec, and the typical voltage is around 8kV. Two degradations of current-voltage (I-V) characteristics occurred depending on ESD-voltage. The minor degradation at the early stage was caused only in the low-current region by a slight ESD pulse (=0.4kV). In contrast, the major degradation occurred over the wide current range of 1μ to 1mA by a highly ESD-voltage (= 8kV). The failure of Schottky barriers by ESD was produced partially in the microstructure. The large degradation was probably caused by the extension of region of broken barriers. The properties of barriers among boundaries and the microstructure play a crucial role in the degradation. In addition, using C-V analysis was found to be extremely valuable for the detection of degradation in MLCV than I-V property.