Pure, 0.1, 0.5 and 1 mol% Tm-doped YAP single crystalline scintillators were grown by the μ-PD method. The XRD analysis confirmed the lattice constants decrease with the Tm concentration. In the transmittance measurement, the absorption bands due to the Tm 3+ 4f–4f transitions were observed at 265, 360, 485, 690 and 800 nm and they were ascribed to the transition from the 3H 6 ground state to its 1I 6, 1D 2, 1G 4, 3F 3 and 3H 4 excited states, respectively. Strong emission peak due to the 1I 6– 3F 4 transition of Tm 3+ appeared at 350 nm under X-ray irradiation. The photoluminescence decay time constants related to this transition were evaluated to be from 15.3 to 17.3 μs and the scintillation decay time constants under gamma-ray excitation were estimated to be from 17.5 to 18.8 μs. The Tm 1% doped crystal exhibited the highest light yield of 15, 100 ± 1500 photons/MeV when excited by 137Cs gamma-ray radiation.