A BLU module has a plurality of BLUs assembled together. Each of the BLUs includes a light guide, a light source structure adjacent to the light guide, and a heat conduction member contacting the light source structure. The light guide has a light-emitting surface arranged on a top thereof and a bottom surface opposing the light-emitting surface, the light-emitting surface being larger than the bottom surface so that that light guide defines a first cavity formed therebetween. The light source structure is received in the first cavity. The light guide in two adjacent BLUs of the module defines a second cavity opposite the first cavity for receiving an adjacent heat conduction member of a prior adjacent light guide. The module is provided with a variable size with the BLUs assembled together to meet requirements.