Thick liquid phase epitaxial YIG (Y 3Fe 5O 12) films which had once grown on Gd 3Ga 5O 12 substrates from R 1 = [ Fe 2 O 3 ]/[ Y 2 O 3] = 14.0 and R 3 = [ PbO]/[ B 2 O 3] = 15.0 melts, dissolved into the melt during the prolonged time dipping process, while spontaneous nucleation of orthoferrite took place. Nevertheless, thin (15 μm) YIG films were available by short time dipping without orthoferrite nucleation. The phase diagram of the PbO-B 2O 3-Y 2O 3-Fe 2O 3 system was modified and had a narrower primary phase region of YIG than that reported by Robertson et al. The fact that thin garnet films are available even from the orthoferrite melt by short time dipping is attributed to a metastable behavior of LPE garnet films, which results from difference in the Gibbs free energy associated with nucleation between bulk orthoferrite and garnet films and from similar values of free energy of formation for both orthoferrite and garnet.