ZnO varistors are characterized by the features of excellent nonlinearity and surge withstand capability. In this paper, in order to investigate the use of ZnO varistor as surge absorption device in low voltage, metal oxide material($TiO_2$) was selected as control material of grain growth. Samples of ZnO varistors were fabricated with varying the contents, and then the microstructures and V-I characteristics were measured. It was observed by SEM that the mean grain size increased with the increase of the additive. From the measurement of V-I characteristics, it was observed that according to the increase of the quantity of $TiO_2$ as additive, the operating voltage was lowered.