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Abstract: Absorption spectrum from 400 to 2000nm and upconversion fluorescence spectra under 940nm pumping of YAG single crystal codoped with 5at.% Yb3+ and 4at.% Tm3+ were studied at room temperature. The blue upconversion emission centered at 483nm corresponds to the transition 1G4 → 3H6, the emission band around 646nm corresponds to the transition 1G4 → 3F4 of Tm3+. Energy transfer from Yb3+ to Tm3+ is mainly nonradiative and the transfer efficiency was experimentally assessed. The line strengths, transition probabilities and radiative lifetimes of 1G4 level were calculated by using Judd–Ofelt theory. Gain coefficient calculated from spectra shows that the upconversion corresponding with transitions 1G4 → 3H6 in YAG doped with Yb3+ and Tm3+ is potentially useful for blue light output. [Copyright &y& Elsevier]