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The microwave dielectric characteristics of (1-x)(Al1/2Ta1/2)O2-x(Mg1/3Ta2/3)O2 (0 x 1.0) ceramics were investigated by crystal structure, variations of ionic polarizability, and microstructures. As x increased, (1-x)(Al1/2Ta1/2)O2-x(Mg1/3Ta2/3)O2 transformed to tetragonal structure. Because the ionic radius of [Mg1/3Ta2/3]4+ was slightly bigger than one of [Al1/2Ta1/2]4+, the cell parameters increased with increase of (Mg1/3Ta2/3)O2 concentration and coincided with prediction of the molecular additivity rule. As x increased, the compositions revealed ordered phase and were of single phase above 60 mol%. The increase of the ordered phase and grain size enhanced the Q and when ordering was completed at x over 0.6, the grain size was major factor for the increase in the Q. Though the grain size increased, however, the porosity deteriorated the Q. Therefore, the Q depended on the order/disorder, the porosity, and the grain size in regular order.