The dependences of phase transition and luminescence properties upon both Eu3+ concentration and heating temperature in air were investigated for GdTiNbO6:Eu3+ fine crystals formed hydrothermally at 240 degrees C as aeschynite-type EuxGd1-xTiNbO6 solid solution. The as-prepared aeschynite solid solution in the concentration range x >= 0.70 did not show the phase transformation from an aeschynite- to an euxenite-type structure caused by heating at 1300 degrees C for 1 h in air. The present study suggested that the phase boundary between the aeschyniteand euxenite-type RETiNbO6 in the EuTiNbO6-GdTiNbO6 system, where RE was a rare earth, lay in the average RE3+ ionic radius 0.1061 nm. The intense emissions attributed to the D-5(0) -> F-7(1) and D-5(0) -> F-7(2) transitions of Eu3+ for the as-prepared samples were more improved by heating at 1100-1300 degrees C in air. The material doped with 30 mol% Eu3+ was preferred for UV-B excitation, in contrast, the material with 50 mol% Eu3+ was preferred for visible light excitation.