The crystal structure and microstructure of microwave dielectric (1-x)(Al1/2Ta1/2)O-2-x(Mg1/3Ta2/3)O-2 (0.2 less than or equal to x less than or equal to 1.0) ceramics have been investigated. It is reported in the JCPDS cards that (Al1/2Ta1/2)O-2 and (Mg1/3Ta2/3)O-2 have orthorhombic and tetragonal structure, respectively. (1-x)(Al1/2Ta1/2)O-2-x(Mg1/3Ta2/3)O-2 ceramics had two phases of orthorhombic and tetragonal allow (Mg1/3Ta2/3)O-2 concentration but as (Mg1/3Ta2/3)O-2 concentration increased, (1-x)(Al1/2Ta1/2)O2-x(Mg1/3Ta2/3)O-2 transformed into tetragonal structure. Specimens having tetragonal single phase could be obtained over x = 0.6. As (Mg1/3Ta2/3)O-2 concentration increased, the cell parameters, X-ray density, and the grain size increased. However, apparent density increased from 0.2 to 0.65 concentration of (Mg1/3Ta2/3)O-2 and then decreased over 0.65 concentration of (Mg1/3Ta2/3)O-2. The number and size of pores in grains also decreased from 0.2 to 0.65 concentration of (Mg1/3Ta2/3)O-2 and then increased over 0.65 concentration of (Mg1/3Ta2/3)O-2. It is expected that the relationship between structural characteristics and microwave dielectric characteristics have a good correlation. (C) 2002 Elsevier Science B.V. All rights reserved.