Bi-substituted rare-earth iron garnet thick films have been grown epitaxially as a material for optical isolators. Films with thicknesses in the range of 300-500 microns were obtained without apparent defects using CaMgZr-GGG substrates. In the course of obtaining thick films of good quality, possible mechanisms for crystal defects, lattice mismatch, and bending behavior of the thick film at the growth temperature have been studied. The temperature dependence of Faraday rotation has also been studied with various combinations of rare-earth ions on the c-site. Films of (YbTbBi) ₃Fe₅O₁₂ show excellent characteristics of the Faraday rotation at the wavelength of 1.3 micron. The rotation is -1800 deg/cm; insertion loss is 0.3 dB; isolation is 42 dB; and the temperature dependence of Faraday rotation is as small as 0.060 deg/K at room temperature. These materials also show good qualities for isolators at the wavelength of 1.55 micron. This technology will lead to the efficient production of the optical isolator used in optical systems.