A backlight unit includes an optical sheet, a receiving member configured to receive the optical sheet, the receiving member having a support surface supporting the optical sheet, an extension configured to protrude from a side surface of the optical sheet, an inner surface position defining portion configured to define a location of the optical sheet, the inner surface position defining portion being substantially perpendicular to the support surface of the receiving member and facing an inner surface of the extension, and an outer surface position defining portion substantially perpendicular to the supporting surface of the receiving member and facing an outer surface of the extension, wherein a first distance between the inner surface position defining portion and the inner surface of the extension is less than a second distance between the outer surface position defining portion and the outer surface of the extension.