Luminescent fine crystals of Y1.00-xEuxTiNbO6, x = 0-1.00 with high crystallinity were directly synthesized by mild hydrothermal method from weakly basic precursor solution mixtures of YCl3, EuCl3, TiOSO4, and NbCl5. Orthorhombic aeschynite-type crystals in the range of 0.5-2.0 mu m consisting of crystallites with 33-91 nm based on a complete solid solution in the YTiNbO6-EuTiNbO6 system were hydrothermally formed at 240 degrees C for 5 h. A single phase of as-prepared aeschynite-type structure was maintained in all the (Y, Eu) TiNbO6 solid solution after heating at temperatures up to 1000 degrees C for 1 h in air. In the range of composition x <= 0.6, the as-prepared aeschynite-type (Y, Eu) TiNbO6 solid solutions transformed to a single phase of euxenite structure after heat treatment at temperatures higher than 1100 degrees C-1200 degrees C. The as-prepared (Y, Eu) TiNbO6 fine crystals in the range of composition x = 0.75-0.90 showed the strongest luminescence in the red spectral region: strong red (D-5(0)-> F-7(2) transition of Eu3+) and weak orange light (D-5(0)-> F-7(1)) line spectra among the as-prepared and heat-treated samples. Another wet chemical synthesis route confirmed the advantage in directly synthesizing the (Y, Eu) TiNbO6 crystals through this hydrothermal method because heating at 1200 degrees C for 1 h in air was necessary for obtaining crystalline (Y, Eu) TiNbO6 with sufficient luminescence intensity in a composition Y0.70Eu0.30TiNbO6 from amorphous powders that were formed via co-precipitation method.