A technique is proposed for determining the dispersion and measureing the thickness of epitaxial garnet films. Use is made fully of the transmission spectrum obtained by spe-ctrophotometer. When the wave length is greater than 2μm so that the absorption and dispersion become negligible, the refractive index at the corresponding wave length may be calculated from the extrema of the transmittance. According to the condition of in-terferrence, the order of interferrence is calculated and deduced for all interferrence extrema. The refractive indices of the film at the wave lengths corresponding to the interferrence extrema may be calculated. Finally the coefficience of Cauchy's dispersion formula are evaluated by the method of least squares, thus the dispersion relation is obtained for this film. Then, in making use of the dispersion relation, the thickness of the film may be calculated.