A fundamental parameter XRF method for the determination of thickness and composition of a magneto-optic disk was studied. The measured Al Kαlins was used to calculate Al layer, while both Si3N4 layers were measured by total Si Kα itensities to discriminate each Si3N4 layer. A fundamental parameter method was used for calculations of magneto-optic layers by Fe Kα, Co Kα and Tb Lα Lines. Equations can be evaluated readily by a computer. The result is in good agreement with the standard sample.