A backlight unit of the invention is reduced in thickness, weight and manufacturing costs but improved in heat releasing efficiency. In the backlight unit, a flexible printed circuit board has at least one through hole perforated therein. An LED package is disposed on a top portion of the flexible printed circuit board corresponding to the through hole. The backlight unit of the invention employs the flexible printed circuit board in place of a metal printed circuit board as a means to conduct current to the LED package. This produces a slimmer and lighter backlight unit and also saves manufacturing costs. In addition, the LED package is directly bonded onto a bottom plate by a heat conducting adhesive, thereby ensuring heat generated from the LED package to be released more quickly.