Bi-substituted rare-earth iron garnets, R_{3− x}Bi _xFe₅O₁₂ (Bi:RIG), where R represents one of the rare-earth elements, exhibit the excellent magneto-optical (MO) properties that increase with Bi content x. In addition, magnetic properties of Bi:RIGs, such as the magnetization, the magnetic anisotropy, and the magnetostriction, could be controlled by choosing rare-earth elements. In this paper, we report on R_0.5Bi_2.5Fe₅O₁₂ (Bi2.5:RIG, R = Pr, Sm, and Eu) thin films on Gd₃Ga₅O₁₂ (GGG) (100) single crystal substrates prepared by the metal–organic decomposition method. XRD analysis reveals that Bi2.5:RIG thin films are grown along the same orientation with GGG substrates, and their lattice constants are dependent on the ionic radii of the rare-earth ions. MO measurements show that Faraday spectra of the Bi2.5:RIG thin films have a typical spectral structure observed for Bi:RIGs. The magnetic anisotropy constants, the uniaxial magnetic anisotropy energy K_u, and the magnetocrystalline anisotropy energy K₁ of Bi2.5:RIG (R = Y, Pr, Nd, Sm, and Eu) thin films are investigated by using the ferromagnetic resonance measurement.