We have studied the crystallization of the yttrium - iron garnet (Y3Fe5O12, YIG) polycrystalline phase in thin films fabricated by means of pulsed laser deposition . Films were deposited on MgO substrates in vacuum, in argon, and in oxygen. A subsequent post-deposition heat treatment (annealing) was done at 800°C in air. We have shown that the crystallization of YIG was precluded by co-existent parasitic phases present in the as-deposited films. Specifically, the growth of the parasitic phase needs to be suppressed in order to get a single-phase polycrystalline YIG. Lowering the substrate temperature has been shown to be a simple and efficient way to suppress the growth of parasitic phase and to obtain good quality YIG films after thermal treatment. This procedure has been demonstrated to be successful even when the YIG films were grown in vacuum and their composition was significantly out of stoichiometry.