The microwave dielectric properties and the microstructures of the (1 − x)Mg 4Nb 2O 9– xSrTiO 3 ceramic system prepared by the conventional solid-state route were investigated. SrTiO 3 was employed as a τ f compensator and was added to Mg 4Nb 2O 9 to achieve a temperature-stable material. Corundum-structured Mg 4Nb 2O 9 and perovskite-structured SrTiO 3 were coexisted and the two-phase system was confirmed by the XRD and EDX analysis. Dielectric properties are correlated to the sintering temperature and the compositional ratio of the specimens. Although the ɛ r of the specimen could be boosted by increasing amount of SrTiO 3, it would instead render a decrease in the Q × f. The τ f value is strongly correlated to the compositions and can be controlled through the existing phases. In fact, τ f could be adjusted to near-zero by mixing 60 mole% Mg 4Nb 2O 9 and 40 mole% SrTiO 3. A dielectric constant ( ɛ r ) of 21.02, a high Q × f value of 112,000 GHz (measured at 9.7 GHz) and a temperature coefficient of resonant frequency ( τ f ) of 1.6 ppm/°C were obtained for 0.6Mg 4Nb 2O 9–0.4SrTiO 3 sintered at 1300 °C for 4 h. It is proposed as a very promising dielectric material for low-loss microwave and millimeter wave applications.