We report the recrystallization of amorphous Yttrium Iron Garnet (YIG) thin films by annealing in an argon atmosphere. Our results show that the amorphous film transforms into a fully epitaxial layer. Compared to annealing in oxygen, the argon atmosphere has no significantly deteriorating influence on the structural and magnetic properties of the YIG thin films. In ferromagnetic resonance experiments, low damping and narrow linewidth can be obtained. For a 65?nm thick layer, a damping constant of ??=?(1.61?±?0.25) × 104 is found and the linewidth at 9.6?GHz is as small as 2.26?±?0.10?Oe. These values are comparable to the best results for YIG thin films grown at high temperature pulsed laser deposition and almost as good as for thin YIG films deposited at room temperature and annealed in oxygen. Annealing in vacuum or growth in argon atmosphere, however, results in non-magnetic material.