The microstructure and electrical properties of varistor ceramics, which are composed of (99.5-x-y)ZnO+xPr(6)O(11)+yCoO+0.5Dy(2)O(3) system, were investigated with Pr6O11/CoO mole ratio (x/y = 0.5/0.5, 0.5/1.0,1.0/0.5, 1.0/1.0) and sintering temperature. The density of varistor ceramics with Pr6O11 = 1.0 was almost constant with sintering temperature, whereas it was increased noticeably in Pr6O11 = 0.5. Increasing Pr6O11 content enhanced the densification for any CoO content and the density was greatly affected not by COO content but by Pr6O11 content. The varistor ceramics with Pr6O11/CoO = 0.5/1.0 exhibited a higher nonlinearity than any other composition ratios. In particular, the varistor ceramics sintered at 1350 degreesC. exhibited the best electrical properties, with nonlinear exponent of 37.8, leakage current of 7.6 muA, and dissipation factor of 0.059. It was found that Pr6O11/CoO ratio greatly affects various characteristics of varistor ceramics. (C) 2003 Elsevier Science B.V. All rights reserved.