A window panel includes: a substrate having a thickness of less than 100 μm and including a first surface and a second surface facing each other in a thickness direction; first ions dispersed in the substrate and each having a first ion radius; and second ions dispersed in the substrate and each having a second ion radius which is greater than the first ion radius, wherein the second ions generate a compressive stress, the compressive stress is reduced with depth from the first surface or the second surface within a range from the first surface or the second surface to a ½ point of the thickness.