A backlight unit including a frame having a bottom area and a sidewall area extended from the bottom area; a single substrate located in on the bottom area of the frame, a plurality of optical assemblies being mounted on the single substrate; a reflection sheet located the frame and configured to reflect light emitted by the optical assemblies; and an optical sheet located over the reflection sheet. Further, the reflection sheet a first sheet area corresponding to the bottom area of the frame; a second sheet area corresponding to the sidewall area of the frame; and a cut portion cut from the first sheet at one portion abutting on one side of the single substrate.