The effect of Sr on the microstructure and nonlinear electrical properties of the (Co, Nb)-doped SnO2 varistors was investigated. The breakdown volta ge of the SnO2-based varistors increased significantly from 240 to 1482V/mm w ith increasing SrCO3 concentration from 0 to 1.50 mol%. Measurement of the ba rrier height at grain boundaries reveal that the significant decrease of the SnO2 grain size with increasing SrCO3 concentration from 0 to 1.50 mol%, is the reason of the enhancement of the breakdown voltage. The origin for the reduction of SnO2 grain size with increasing SrCO3 concentration was ex plained. The 1.50 mol% SrCO3-doped SnO2 varistor with ultrahigh breakdown vo ltage (1482V/mm) and larger nonlinear coefficient α(21.4) is a candidate used in the ultrahigh voltage protection system.