In this research, effects of two-step sintering parameters, such as first and second step sintering temperatures (T-1 and T-2, respectively) and soaking time in the second step, on microstructure and electrical properties of ZnO varistors have been studied. In addition, slow rate of grain growth in the second step of sintering has also been investigated. Near fully dense (higher than 99%) and sub-micron grain size (1-0.75 mu m) ZnO varistors have been fabricated by different two-step sintering cycles. Decreasing T-1 and T-2 result in longer necessary time in the second step to reach near full density with smaller final grain size. The superior electrical properties of varistors (V-B: 2050-1022 V/mm and a: 96-60) are attributed to very fine microstructure and high concentration of ZnO-ZnO grains direct contacts. Furthermore, lower T-1 and T-2 with shorter soaking time improve electrical properties of the ZnO varistors. (C) 2010 Elsevier B.V. All rights reserved.