A backlight module has a frame, on which a reflective film, lamps and a diffusive plate are provided in sequence. A plurality of positioning devices each have a base and two rings. The base has two channels, two slots and a post on a top thereof. The lamps are fitted to the rings respectively and the rings are mounted in the slots of the base respectively to rest the lamps in the channels. The base of the positioning devices are secured on the bottom plate of the frame and the posts thereof are against the diffusive plate, such that the lamp is suspended and the relative locations of the lamps, the diffusive plate and the reflective film are fixed.