Abstract: A heat-dissipating device for a back light source for a flat panel display includes at least one back light unit, a circuit board, and at least one heat-dissipating board. The back light unit includes a lighting element and a thermally conductive portion connected thereto for absorbing heat generated by the lighting element. The circuit board includes at least one assembling hole and is electrically connected to the back light unit to form a back light module. The heat-dissipating board is mounted to a rear side of the heat-dissipating board. The thermally conductive portion of the back light unit is extended through the assembling hole of the circuit and directly in contact with the rear side of the heat-dissipating board, allowing the heat-dissipating board to absorb and thus dissipate the heat generated by the back light module.