The microwave dielectric properties and the microstructures of the (1-x)MgTiO 3 -xCaTiO 3 ceramic system were investigated. With partial replacement of Mg by Co, dielectric properties of the (1-x)(Mg 0 . 9 5 Co 0 . 0 5 )TiO 3 -xCaTiO 3 ceramics can be promoted. The microwave dielectric properties are strongly correlated with the sintering temperature. At 1275 o C, the 0.95(Mg 0 . 9 5 Co 0 . 0 5 )TiO 3 -0.05CaTiO 3 ceramics possesses excellent microwave dielectric properties: a dielectric constant r of 20.3, a Qxf value of 107000 ( at 7GHz) and a τ f value of -22.8ppm/ o C. By appropriately adjusting the x value in the (1-x)(Mg 0 . 9 5 Co 0 . 0 5 )TiO 3 -xCaTiO 3 ceramic system, zero τ f value can be achieved. With x=0.07, a dielectric constant γ of 21.6, a Qxf value of 92000 (at 7GHz) and a τ f value of -1.8ppm/ o C was obtained for 0.93(Mg 0 . 9 5 Co 0 . 0 5 )TiO 3 -0.07CaTiO 3 ceramics sintered at 1275 o C for 4h.