The crystalline phase and the microwave dielectric properties of the Mg(Ta1-xNbx)(2)O-6 ceramics have been investigated. Combining the positive temperature coefficient of the resonant frequency (tau(f)) material (MgTa2O6, tau(f) = +30 ppm/degrees C) with the negative one (MgNb2O6, tau(f) = -70 ppm/degrees C) can produce the composite with tau(f) similar to 0 ppm/degrees C. The crystal structure of pure MgTa2O6 ceramic is the tetragonal structure. For x = 0.15, the crystal structure of the solid solution was the coexistence of the tetragonal structure and orthorhombic structure. Solid solutions with 0.25 <= x <= 1 exhibit the orthorhombic structure, which is like the structure of pure MgNb2O6 ceramic. The sintered morphologies of Mg(Ta1-xNbx)(2)O-6 ceramics gradually change from the disk-typed grains (x = 0 and 0.15) to the disk-typed and bar-typed grains coexist (x = 0.35, 0.5, and 0.7), and then reveal the bar-typed grains (x = 0.85 and 1). The densities, dielectric constants (epsilon(r)) and tau(f) values of Mg(Ta1-xNbx)(2)O-6 ceramics decrease with the increase of the MgNb2O6 content. The quality factor (Q x f) reaches the minimum value at x = 0.15, and then increases with the increase of the MgNb2O6 content. The Mg(Ta1-xNbx)(2)O-6 ceramic with x = 0.25 sintered at 1450 degrees C exhibits a minimum tau(f) value of -0.7 ppm/degrees C. (C) 2006 Elsevier Ltd. All rights reserved.