The microwave dielelectric properties and the microstructures of the (1-x)MgTiO3- xCaTiO(3) ceramic system were investigated. With partial replacement of Mg by Co, dielectric properties of the (1-x)(Mg0.95Co0.05)TiO3-xCaTiO(3) ceramics can be promoted. The microwave dielectric properties are strongly correlated with the sintering temperature. At 1275degreesC, the 0.95(Mg0.95Co0.05)TiO3-0-05CaTiO(3) ceramics possesses excellent microwave dielectric properties a dielectric constant epsilon(r) of 20.3, a Q x f value of 107 000 (at 7 GHz) and a tau(f) value of -22.8 ppm/degreesC. By appropriately adjusting the x value in the (1-x)(Mg0.95Co0.05)TiO3-xCaTiO(3) ceramic system, zero tau(f) value can be achieved. With x = 0.07, a dielectric constant epsilon(gamma) of 21.6, a Q x f value of 92 000 (at 7 GHz) and a tau(f) value of -1.8 ppm/degreesC was obtained for 0.93(Mg0.95Co0.05)TiO3-0.07CaTiO(3) ceramics sintered at 1275degreesC for 4 h. (C) 2002 Elsevier Science Ltd. All rights reserved.