The effects of a Bi2O3-B2O3 frit (denoted as BiBF) on the densification, phase transformation, microstructure, and nonlinear electrical properties of ZnO-Bi2O3-TiO2-Co2O3-MnCO3 (ZBTCM)-based varistors have been investigated. The varistor ceramic with the addition of > 2 wt% BiBF can be sintered at 900 degrees C and 96.3% of the theoretical density was achieved. SEM revealed that the microstructure of the varistor ceramics with 2 wt% BiBF is uniform, with an average grain size of 13.5 mu m. No porosity is observed. The typical ZnO-Bi2O3-TiO2-based varistor phase compositions were detected for all varistor ceramics but the formation of a secondary phase, Bi4B2O9, became apparent when 4 wt% BiBF was added. The Bi4B2O9 phase crystallized from the Bi2O3-B2O3 frit, acting as a potential barrier at the grain boundary, increases the density of interface states N-s, barrier height (b), and plays an important role in improving the electrical properties of the varistor ceramics. However, a large amount will be detrimental. Optimum properties were exhibited for the varistor ceramics with 2 wt% BiBF sintered at 900 degrees C, with E-1 mA=124.9 V/mm, alpha=46.2, and J(L)=0.2 mu A/cm2. The reactivity of the varistor ceramic with Ag was also studied. The excellent electrical properties and low sintering temperature, coupled with chemical compatibility with Ag, suggest that Bi2O3-B2O3 frit sintering aids for ZnO-Bi2O3-TiO2-Co2O3-MnCO3 (ZBTCM)-based varistors have great potential for using the base metal Ag as inner electrodes for low-voltage MLV fabrication.