A backlight unit. The backlight unit is employed in a display device having a main screen and a sub screen. The backlight unit comprises a first light-emitting unit and a second light-emitting unit. When the display device uses the sub screen, the first light-emitting unit provides a first light to the sub screen. When the display device uses the main screen, the first and the second light-emitting units respectively emit a second light and a third light, and a first aggregate light comprising the second light and the third light is provided to the main screen.