Tm-doped Y 3 Al 5 O 12 (YAG) single crystals with different Tm concentrations (0.1, 0.5, 1.0, and 2.0%) were grown using the floating zone method. X-ray-induced scintillation spectra, scintillation decay curves, and γ-ray-induced pulse height spectra of the obtained Tm-doped YAG single crystals were investigated. All the samples showed emission peaks due to the 4f-4f transitions of Tm 3+ ions, and the decay times were around 40-20 μs under X-ray excitation. The 0.5% Tm-doped YAG sample showed the highest light yield under 662 keV γ-ray excitation among the samples. The optimal dopant concentration of Tm-doped YAG single crystals was confirmed to be 0.5%.