The influence of the amount of Pr6O11 additions on the microstructure and electrical properties of varistors ceramics in the ZnO-Bi2O3 system was investigated. Samples with a low level of Pr6O11 (0.1wt %) have high microstructural homogeneity, which enhances the nonlinear coefficient greatly, and decreases the leakage current without change of voltage ratio. When the Pr6O11 content reached 7wt%, the ZnO grain growth was restricted and the threshold voltage was improved from 275v/mm to 440v/mm. The additive of Pr6O11 changed the process of creating spinel phase, which came from the decomposition of pyrochlore phase. This type of small size phase has more dragging force on the ZnO crystal, which make the whole materials more uniform and compact.