Up-conversion spectra of Tm 3+ and Ho 3+ ions in LiYF 4 crystals have been investigated at various temperatures between 10 and 320 K. The photoluminescence is investigated under excitation with Xe-lamp and laser diode. In addition to blue up-converted emission of Tm 3+ and Ho 3+ and green up-converted Ho 3+ emission, anti-Stokes emission bands are observed at 687 and 703 nm under excitation in the 3H 4 state of Tm 3+ with 785 nm laser diode. These bands are observed above 200 K, and their intensities increase exponentially with increasing temperature. They are attributed to endothermic Tm 3+ emission due to the transition to the 3H 6 ground state from the upper 3F 3 state, which is thermally populated from the 3H 4 state. Discussion is given on the optical process of green up-converted Ho 3+ emission, which is generated by the 785 nm laser diode excitation.