Purpose: To detect abnormalities of the optic nerve in patients with thyroid orbitopathy using diffusion tensor MRI. Methods: Twenty-eight patients with Graves orbitopathy prospectively underwent diffusion tensor imaging scanning. A full ophthalmic examination including visual acuity, intraocular pressure, fundoscopy, and visual field analysis was performed. Clinical activity scores were also calculated. Fractional anisotropy (FA) and mean diffusivity values of the patients were compared with age and sex-matched healthy control subjects. Results: The mean FA values were decreased and mean diffusivity values were increased significantly in patients with Graves orbitopathy compared with the control subjects (p < 0.001). There was a strong reverse correlation between the FA levels and the visual fields in 4 quadrants of the optic nerve. In addition, there was a strong correlation between the degree of proptosis and the FA values in both eyes. The mean diffusivity levels were also correlated with changes in the visual field and the degree of proptosis. Conclusion: FA and mean diffusivity levels measured with the diffusion tensor imaging of the thyroid orbitopathy patients were affected. The changes in diffusion tensor imaging were also correlated with the ophthalmologic tests of the patients.