The microwave properties and microstructures of (ZnMg)TiO3-based dielectric prepared by conventional solid-state method were investigated as functions of CaTiO3 and CaO-B2O3-SiO2 additions. The effects of CaTiO3 on the crystal phase and the evolution of microstructure of (Zn0.65Mg0.35)TiO3 were studied. The result indicated that CaTiO3 secondary phase coexists with (ZnMg)TiO3 main phase in the ZMT-CT ceramics, which confirmed by EDS analysis. Because of CaTiO3 with large τf value (τf = 800 ppm/°C), the temperature coefficient of resonant frequency (τf) of ZMT-CT with biphasic structure was adjusted to near zero value. The microwave properties of (Zn0.65Mg0.35)TiO3 ceramics doped with 5wt% CaTiO3 sintered at 1150°C were ε ≈ 24, τf ≈ ±10 ppm/°C, Q×f > 45,000 GHz. Further, it was found that the CaO-B2O3-SiO2 additive could successfully reduce the sintering temperature of (Zn0.65Mg0.35)TiO3–CaTiO3 ceramics from 1150 to 950°C, and significantly improve the densification of this system, which were densified below 1000°C. This was due to the formation of liquid phases during the sintering observed by SEM. The (Zn0.65Mg0.35)TiO3–0.05CaTiO3 dielectrics with 1 wt% CaO-B2O3-SiO2 sintered at 950~1000°C exhibited the optimum microwave properties: ε ≈ 22, Q×f ≈ 20,000 GHz and τf ≈ ±10 ppm/°C.