A backlight module is provided. The backlight module has a reflective chamber and a light source. The reflective chamber includes a bottom reflector, a top reflector, at least a light entrance, and a plurality of light emitting openings disposed on the top reflector. The light source is disposed corresponding to the light entrance. The number of the light emitting openings per unit area in a region near the light source is less than the number of the light emitting openings per unit area in another region far from the light source.