The change of growth rate with supercooling and growth time was investigated in the thick film growth of (TbBi)₃(FeAlGa)_5O_(12) garnet single crystal. At the degree of supercooling smaller than critical supercooling having maximum growth rate, growth rate was kept constant and the film composition did not change until the supersaturated garnet solute was depleted. After depletion the Bi concentration profile was decreased with the film thickness. At the degree of supercooling larger than critical supercooling, growth rate was decreased with growth time because of nucleation and growth of garnet microcrystals at the Pt crucilble wall. Bi concentration profile was decreased with the film thickness but concentration of Fe, Al, Ga was kept constant. The variation of Bi concentration with the film thickness is proportion to the film growth rate. It is found out that the interfacial supersaturation βi is decreased with growth time even though growth temperature is kept isothermally.