This paper analyses various factors leading to optical attenuation in magneto-optic grauet films and derives expressions of optical absorption coefficient a and optical scattering coefficient γ. Values of a and 7 of (BiTm)3(FeGa)5O12 films were measured on the magnetooptic rotation measurement installation by means of the comparison method of the magneto-optic modulation.By comparison with an ordinary method, it can not only eliminate effect of absorption in a substrate and reflection on the interface between a film and a substrate upon measuring the value of a of garnet films, but also separate optical scattering coefficient γ from optical absorption coefficient a, so that the measured value of a comparatively approaches the true value of optical absorption coefficient of magneto-optic garnet film materials.