TiO2 ceramics doped with 1.0 mol% Ca and different concentrations of Ta were obtained by sintering processing at 1450degreesC. The microstructures, nonlinear electrical behavior and dielectric properties of the ceramics were investigated. The samples have nonlinear coefficients of alpha = 2.0-5.0 and ultrahigh relative dielectric constants which is up to 10(5). Especially, the effects of Ta dopant on the nonlinear electrical characteristics and dielectric properties of the (Ca, Ta)-doped TiO2 ceramics were studied in detail. When the concentration of Ta is 2.0 mol%, the sample exhibits the highest nonlinear coefficient and a comparatively lower dielectric constant. By analogy to a grain-boundary atomic defect model, the effects of Ta and the nonlinear electrical behavior of the TiO2 system were explained. (C) 2003 Kluwer Academic Publishers.