The preparation and magneto-optic properties of YIG thin films were investigated. Ce-substituted yttriumiron garnet(Ce 1YIG) thin films were grown on an amorphous silica substrate by RF magnetron sputtering method. Then the as-grown amorphous Ce 1YIG thin films were annealing in different ambient gas to obtain the crystallized film with magneto-optic effect. The post annealing process was investigated in detail, and the influence of microstructure of films on magntooptic properties was discussed. The results show that the saturation Faraday rotation coefficient θ F of about 0.8deg/μm was obtained for light at a wavelength of 630 nm, and the strong in-plane magnetization was observed. The curie temperature is 220℃ measured by VSM at atmosphere. All these parameters show that Ce∶YIG thin film prepared is suitable for fabrication of magnetooptic waveguide isolator.