The CaCu3Ti4O12-xMgTiO3(x= 0, 0.25, 0.5, 1.0) ceramics have been prepared by a solid-state reaction method. The effects of MgTiO3 doping on the phase structure, microstructure and dielectric properties of CaCu3Ti4O12 ceramics have been investigated. The results indicate that MgTiO3 doping not only reduced the dielectric loss of low frequency range and raised the breakdown voltage but also significantly improved the I-V nonlinearity coefficient. The optimized properties of MgTiO3 doped CaCu3Ti4O12 can be well explained by the uniformity of the grains, the reduction of the average grain boundary thinkness and the enhancement of the grain boundary resistance. Among the CaCu3Ti4O12-MgTiO3 specimens in this work, the CaCu3Ti4O12-0.5MgTiO3 specimen has achieved the best comprehensive properties, which include a dielectric constant (εr) of 53958, dielectric loss (tanδ) of 0.06 at 1 kHz, breakdown voltage (Eb) of 295 V/mm and a large nonlinearity coefficient of 66.3.