Ruthenium (Ru)-doped lithium niobate (LiNbO3) single crystals were grown by the Czochralski method from a congruent melt composition. The color of the Ru: LiNbO3 was red and darkened with increasing Ru concentration. The Ru concentration in the grown crystal gradually decreased along the pulling direction because the effective segregation coefficient of Ru in lithium niobate is greater than one, and also because RuO2 evaporated during the crystal growth period. However, a solubility limit does exist in Ru: LiNbO3 crystals, so the maximum amount of Ru doped into LiNbO3 single crystals is about 0.2 mol%. In addition, the lattice constants of Ru: LiNbO3 on the A- and C-axis decreased with the concentration of Ru in the crystal. An absorption spectra examination of Ru: LiNbO3 showed that there were two absorption peaks around 370 and 530 nm that is within the UV/VIS region. The absorption coefficients should increase, and the absorption edges shift toward longer wavelength as the Ru concentration increases. The OH- absorption spectra confirm that Ru ions may have substituted for both Li vacancies (V-Li) and antisite Nb ions (Nb4+Li) as the Ru doping concentration in the present case is well below the threshold limit.