The present invention provides an electro-optic unit. A lens unit disposed at an upper portion of a display panel has a plurality of lens part. An electro-optic unit is disposed between a display panel and a lens unit, and includes an electro-optic material layer formed as a graded refractive index lens in an electric field. A display device shows a two-dimensional and a three-dimensional image according to a mode of the electro-optic unit. A driving part may form the graded refractive index lens to have the same pitch as the pitch of the lens part. The graded refractive index lens may be formed as a convex lens or a Fresnel lens. The electro-optic unit is displayed to form the Fresnel lens. A driving method enhancing mode conversion velocity of the electro-optic unit is displayed.