The microstructure, electrical properties, and DC accelerated aging characteristics of varistors, which are composed of ZnO-Pr6O11-CoO-Cr2O3-Dy2O3 ceramics, were investigated at various Coo contents. The varistor ceramics was more densified as increasing density in the range of 5.25-5.55 g cm(-1) with increasing Coo contents. The varistor with Coo of 1.0 mol% exhibited the highest nonlinearity, with 66.6 in nonlinear exponent and 1.2 muA in leakage current, and also the highest stability in the light of nonlinear properties and dielectric characteristics. Furthermore, the varistor with Coo of 1.0 mol% exhibited superior DC accelerated aging characteristics to other varistors in nonlinear properties and dielectric characteristics. Finally, it is assumed that the Coo is moderate contents in the range of 1.0-2.0 for this composition. (C) 2004 Elsevier B.V. All rights reserved.