In order to solve the problems of acceptor/donor individual doping in Li2TiO3 system and clarify the superiority mechanism of co-doping for improving the Q value, Mg + Nb co-doped Li2TiO3 have been designed and sintered at a medium temperature of 1260 degrees C. The effects of each Mg/Nb ion on structure, morphology, grain-boundary resistance and microwave dielectric properties are investigated. The substitution of (Mg1/3Nb2/3)(4+) inhibits not only the diffusion of Li+ and reduction in Ti4+, but also the formation of microcracks in ceramics, which promotes the enhancement of Q value. The experiments reveal that Q x f value of Li2TiO3 ceramics co-doped with magnesium and niobium is 113 774 GHz (at 8.573 GHz), which is increased by 113% compared with the pure Li2TiO3 ceramics. And the co-doped ceramics have an appropriate dielectric constant of 19.01 and a near-zero resonance frequency temperature coefficient of 13.38 ppm/degrees C. These results offer a scientific basis for co-doping in Li2TiO3 system, and the outstanding performance of (Mg + Nb) co-doped ceramics provides a solid foundation for widespread applications of microwave substrates, resonators, filters and patch antennas in modern wireless communication equipments.