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Abstract: High-relative-permittivity and low-loss ceramics in the (1− x)Ca(Mg1/3Ta2/3)O3–xCa0.8Sm0.4/3TiO3 system have been prepared by the conventional solid state route and their microwave dielectric properties have been investigated. (1− x)Ca(Mg1/3Ta2/3)O3–xCa0.8Sm0.4/3TiO3 solid solution exhibited orthorhombic crystal structures for all compositions. A rapid grain growth is observed at temperatures higher than 1450°C, which would lead to a decrease in the density and the Q × f of the ceramics. A near-zero temperature coefficient of resonant frequency can be achieved for Ca(Mg1/3Ta2/3)O3 with the addition of Ca0.8Sm0.4/3TiO3. The temperature coefficient of resonant frequency increases with increasing Ca0.8Sm0.4/3TiO3 content and tunes through zero at x =0.45. Specimen using 0.55Ca(Mg1/3Ta2/3)O3–0.45Ca0.8Sm0.4/3TiO3 possesses an excellent combination of microwave dielectric properties: ɛ r ∼44.96, Q × f ∼41,700GHz (where f =6.6GHz, is the resonant frequency) and τ f value of 0ppm/°C. It is proposed as a suitable candidate material for today''s 3G passive components and small-sized GPS patch antennas. [Copyright &y& Elsevier]