The electro-optic index of PLZT thin films was measured by means of modulated ellipsometry. First the refractive index (n) and thickness (d) were measured by reflecting ellipsometry method. Then the electric field was applied on the film, and the change of refraction (δn) by was obtained by transmission ellipsometry method. Finally, the electro-optic index was calculated by the data of n, d, E and δn. This instrument has high sensitivity. So it is suitable to measure the electro-optical property of the thin film.