The effect of annealing on varistor properties of TiO2-Ta2O5-CaCO3 varistor ceramics was studied. TiO2-Ta2O5-CaCO3 varistor ceramics were prepared by the traditional ball mill-molding-sintering method, then these varistor ceramics were annealed at different temperatures. The nonlinear coefficient α and varistor voltage EB of these samples were tested with varistor DC parameters and their microstructures were analyzed by XRD, SEM and STEM. The result shows that annealing at suitable temperature for appropriate time can make the grain grow up slightly, which results uniform grain sizes, less porosity and higher grain density. During annealing, the acceptor ions with larger radius obtain kinetic energy and move towards the grain boundary. The state density of acceptor increases at the grain boundary will improve the nonlinear coefficient α. The number and total area of grain boundaries decrease with grain grown properly, which is helpful to reduce the varistor voltage. At the same time, the densification of grains increases and the resistivity decreases, which further reduces the varistor voltage. When doping concentration is 0.20mol%, and sintering temperature is 1 350 ℃, TiO2-Ta2O5-CaCO3 varistor ceramic annealed at 700 ℃ for 3 h obtains the highest nonlinear coefficient and lower varistor voltage (α=8.6, EB=22.5 V?mm-1), which is better than TiO2-Ta2O5-CaCO3 varistor ceramic unannealed.