When excited by 532 nm photons, the Er3+ ions in KY0.9Er0.1F4 crystal emit photoluminescence (PL) around 520 nm, in addition to previously reported up-converted emission around 410 nm. While the intensity of the latter varies non-linearly with excitation power, the former shows a linear behavior, which implies that an electron in the 520 nm up-conversion process neither requires other electron's assistance nor needs two photons for the up-conversion to occur. We explain this 520 nm up-conversion by a level crossing mechanism which provides the two levels H-2(2)(11/2) and S-4(3/2) with a thermal equilibrium. This is in agreement with the observed increasing up-conversion efficiency with increasing temperature. The thermal equilibrium of the two levels is possible by the proximity of the two levels and also by the long lifetime of the PL even with a short pulsed excitation. (C) 2011 Elsevier B.V. All rights reserved.