A backlight unit adapted to enhance brightness and reduce manufacturing time and costs is disclosed. The backlight unit includes a bottom cover with an opened upper surface, a plurality of light emission diodes arranged on an inner edge of the bottom cover, a light guide plate disposed parallel to the plurality of light emission diodes, and a diffusion sheet disposed on the light guide plate. The diffusion sheet is provided to have an overlapped area with a support main which is combined with the bottom cover. Also, a reflection material layer is formed on an upper surface of the diffusion sheet corresponding to the overlapped area.