ZnO-based varistor ceramics with various amounts of CaSiO3 have been prepared by the conventional solid-state reaction route, and the phase composition, microstructure and electrical properties have been investigated by XRD, SEM, a V-I source/measure unit and impedance spectroscopy (IS). The XRD analyses show the presence of primary phase ZnO and some minor secondary phases. The electrical properties analysis results show that the nonlinear coefficient of the varistor ceramics is in the range of 14.9-30.9, the breakdown voltage in the range of 57.0-168.8 V/mm, and the leakage current in the range of 1.1-31.5 mu A/cm(2). A schematic model for the electrical microstructure of ZnO-based varistor ceramics is proposed with an equivalent circuit of R-a (RgCg)( RgbCgb) by IS.