The effect on microstructure and electrical properties of (Co, Nb)-doped SnO2 varistors upon the addition of Pr2O3 was investigated by scanning electron microscopy and by determining I-V, epsilon-f, and R-f relations. The threshold electric field of the SnO2-based varistors increased significantly from 850 to 2280 V/mm, and the relative dielectric constants of the SnO2-based varistors decreased greatly from 784 to 280 as Pr2O3 concentration was increased up to 0.3 mol%. The significant decrease of the SnO2 grain size, from 4.50 to 1.76 gm with increasing Pr2O3 concentration over the range of 0-0.3 mol%, is the origin for the increase in the threshold voltage and decrease of the dielectric constants. The grain size reduction is attributed to the segregation Of Pr2O3 at grain boundaries hindering the SnO2 grains from conglomerating into large particles. Varistors were found to have a superhigh threshold voltage and a comparatively large nonlinear coefficient alpha. For 0.15 mol% Pr2O3-doped sample, threshold electric field and nonlinear coefficient a were measured to be 1540 V/mm and 61, and for 0.3 mol% Pr2O3-doped sample, V and alpha were 2150 V/mm and 42, respectively. Superhigh threshold voltage and large nonlinear coefficient a qualify the Pr-doped SnO2 varistor as an excellent candidate for a high voltage protection system.