(1 - x)MgTiO3-xCa0.8Sr0.2TiO3 (0.04 <= x <= 0.2, MT-CST) composite ceramics were prepared by the conventional solidstate reaction process. The phase composition, microwave dielectric properties, and microwave dielectric loss mechanisms were studied. Ca0.8Sr0.2TiO3 was employed as a tau(f) compensator for MgTiO3, and they coexisted well without forming any secondary phases. Interestingly, significant dielectric relaxations associated with oxygen vacancy defects were observed in all the MT-CST ceramics through the dielectric-temperature spectra. Thermally simulated depolarization current was therefore conducted to obtain the defects associated with extrinsic dielectric loss mechanisms. The concentrations of both defect dipole [(Ti-Ti') - (V-O(center dot center dot))] and in-grain oxygen vacancies (V-O(center dot center dot)) increased with the increasing x, which could induce microwave dielectric loss consequently. It demonstrated that the behaviors of Q x f were basically influenced by phase composition and defects here. Temperature-stable ceramics can be achieved at x = 0.06, where the microwave dielectric properties were epsilon(r) = 21.19, Q x f = 110 900 GHz (f = 9.295 GHz), and tau(f) = -0.9 ppm/degrees C, respectively.