Dielectric properties of (Mg_(1−x)Co_x)₄Ta₂O₉ (x = 0.01 to 0.09) prepared by solid state reaction were investigated. The maximum properties of the (Mg_(1−x)Co_x)₄Ta₂O₉ (x = 0.05) system were namely, an obtained ε_r value of 12.27, a Q×f value of 440,000 GHz, and a τ_f value of −59 ppm/°C sintered at 1325°C. The theoretical polarizability of (Mg_(1−x)Co_x)₄Ta₂O₉ increased with Co substitution, x. (Mg_(1−x)Co_x)₄Ta₂O₉ with Mg²⁺ substituted by and Co²⁺ not only improved the microwave dielectric properties, but also decreased sintering temperature. From the calculation of the covalency of cation–oxygen bonds, it is found that the covalency of the Co–O bond is lower than that of the Mg–O bond and the difference in the covalency between these cation–oxygen bonds influences the temperature dependence of dielectric constant on the compounds. From our experiment, the best microwave dielectric property happens in (Mg_0.95Co_0.05)₄Ta₂O₉ sintered at 1325°C for 4 h, and it’s Q×f is 440,000 GHz with τ_f = 59.5 ppm/°C.