The effects of Sc 2 O 3 on the properties of (Co, Ta)-doped SnO 2 varistors were investigated. It was found that the nonlinear coefficient presents a peak of α=19.3 for the sample doped with 0.05mol% Sc 2 O 3 . The breakdown electrical field increased from 303 to 1281V/mm and relative dielectric constant decreased from 921 to 117 with increasing Sc 2 O 3 from 0 to 0.09mol%. The variations of breakdown electrical field and relative dielectric constant were mainly attributed to the decrease of SnO 2 grain size, and mechanism of the decrease of SnO 2 grain size was discussed. To illustrate the grain boundary barrier formation of (Sc, Co, Ta)-doped SnO 2 varistors, a modified defect barrier model was introduced.