The optical birefringence of various LPE-grown rare-earth and Bi-substituted iron garnet films are studied. In addition to the stress-induced birefringence, it is shown that there exists growth-induced birefringence ΔnfG in the films containing light rare-earth or Bi ions. In Bi garnet films the ΔnfG disappears on annealing and the thermal behavior of ΔnfG is similar to that of the growth-induced uniaxial magnetic anisotropy constant KuG. Furthermore, the BiG content dependence of ΔnfG is similar to that of KuG. As a result, it is suggested that the ΔnfG and KuG originate in the same microscopic structure which is of an anisotropic character. It is pointed out that the positive sign and the magnitude of ΔnfG in Bi garnet films are effective for the phase matching between TE and TM modes in a planar waveguide.