In order to study the origin of perpendicular magnetic anisotropy in Bi-substituted iron garnet thin films, a series of samples with composition YBi2Fe5-xGaxO12 (0.36/≪x/≪0.97) was prepared by rf sputtering onto several substrates with various thermal expansion coefficients. The stress (Â¿) was calculated from the difference in thermal expansion of the film and substrate. The anisotropy constant (Ku) of the films increased linearly with Â¿ in the range from Â¿ = Â¿9.0Ã109 erg/cm3 to 3.0Ã109 erg/cm3. The Ku decreased linearly with x in the composition range studied. The experimental results are discussed and compared with results for Al-substituted films.