The microwave dielectric properties of (1 - x)(Al1/2Ta1/2)O-2-x(Mg1/3Ta2/3)O-2 (0 less than or equal to x less than or equal to 1.0) ceramics were investigated. (Al1/2Ta1/2)O-2 and (Mg1/3Ta2/3)O-2 had orthorhombic and tetragonal structure, respectively. As (Mg1/3Ta2/3)O-2 concentration increased, (1 - x)(Al1/2Ta1/2)O-2-x(Mg1/3Ta2/3)O-2 transformed to tetragonal structure. Specimens having a tetragonal single phase could be obtained for x > 0.6. As (Mg1/3Ta2/3)O-2 concentration increased, the grain size, dielectric constant (epsilon (r)) and quality factor (Q) significantly increased, and the temperature coefficient of resonant frequency (tau (f)) changed from a negative to a positive value. A tau (f) of 0 ppm/degreesC was realized at x = 0.65 and the Qf(0) value and epsilon (r) for this composition were 112470 GHz and 26.1, respectively. The relationship between microstructures and dielectric properties was also investigated.