Ba(Mgl-xCox)TaO3[BMCT] ceramics were prepared by the conventional mixed oxide method. The ceramics were sintered at the temperature of 1525~$1625^{circ}C$ for 5hr. in air. The crystal structure of BMCT ceramics was investigated by the XRD. The microstructure of the specimens were observed by SEM. The Microwave dielectric properties of BMCT specimens were investigated as a function of composition and sintering temperature. All BMCT ceramics sintered over 1575$^{circ}C$ were showed a polycrystalline complex perovskite structure. The density of BMCT (90/10) specimen sintered at $1575^{circ}C$ was 7.75g/㎤. As the Co contents decreased, the ordering parameter of B-site in BMCT increased. In the case of the BMCT(90/10) ceramics sintered at $1575^{circ}C$ for 5 hours, dielectric constant, quality factor and temperature coefficient of resonant frequency for microwave dielectrics application were a good value o( 25, 17, 845 at 10㎓ and +2.4 ppm/${circ}$, respectively.