CaCu2.95Zn0.05Ti4-xSnxO12 ceramics with x = 0.05−0.30 were successfully prepared by a conventional solid−state reaction method. The lattice parameter of the sintered ceramics increased with increasing Sn4+ doping concentration. Scanning electron microscopy images show a good dispersion of all major and dopant elements. A homogeneous and fine microstructure with a grain size of ~1.8−2.1 μm was clearly observed in the CaCu2.95Zn0.05Ti4-xSnxO12 ceramics. Substantially enhanced varistor properties for CaCu3Ti4O12 were achieved by co−doping with Zn2+ and Sn4+ ions. Excellent varistor properties with a breakdown electric field of ~1.36 × 104 V/cm and nonlinear coefficient of ~38.26 were achieved in Cu2.95Zn0.05Ti3.70Sn0.30O12 ceramics. The grain boundary resistance was closely correlated with the breakdown electric field and conduction activation energy at the grain boundary, which were increased with increasing Sn4+ doping. The dielectric properties with a dielectric permittivity of ~2,100−5,200 and low loss tangent of ~0.028−0.092 were observed in all CaCu2.95Zn0.05Ti4-xSnxO12 ceramics. The modification of the intrinsic electrical properties of the grain boundaries in Zn2+ and Sn4+ co−doped CaCu3Ti4O12 is the primary reason for the substantially enhanced varistor and improved dielectric properties.