Refractive‐index dispersion of magnetic garnet films of composition (YSmLuCa)3(GeFe)5O12is reported in a wavelength range of 0.55–1.5 &mgr;. The dispersion was calculated from the film’s optical interference data and thickness value which was accurately determined by the technique of thin‐film optical waveguide measurement. The calculated index values at the interference fringe locations were fitted to a two‐term Sellmeier formulation. The accuracy was estimated to be better than 0.3%.