The effect of Mg loading on the high-frequency tunable properties and dielectric loss of Ba0.8Sr0.2Ti1-xMgxO3 (BSTM) ceramics was investigated. Variation in the lattice parameters and the 90 degrees domain configuration with Mg loading indicated a decrease in the tetragonal distortion. Additionally, the 90 degrees domain size decreased slightly with a low Mg loading, up to 0.1 mol %, resulting in a higher domain-wall density compared with the non-doped specimen. The 0.075 mol% Mg-loaded BSTM ceramic exhibited the highest tunability; this was attributed to the domain-size effect. The loss tangent (tan delta) roughly decreased with Mg loading, due to loaded oxygen vacancies. The maximum figure of merit value (FOM = tunability/tan delta) at 10 MHz was achieved for the 0.075 mol% Mg specimen, twice that of the non-doped specimen, due to an increase in the tunability and a decrease in the loss tangent with Mg loading. (C) 2015 The Japan Society of Applied Physics