A backlight module includes a light guide plate, a light source, a prism sheet and a reflective structure. The light guide plate includes a light emitting surface, a bottom surface opposite to the light emitting surface, a light incident surface connected with the light emitting surface and the bottom surface and a plurality of micro structures located on the bottom surface or the light emitting surface. The light source is disposed beside the light incident surface of the light guide plate. The prism sheet is disposed above the light emitting surface of the light guide plate, and includes a plurality of prisms facing the light emitting surface. The reflective structure is disposed under the bottom surface of the light guide plate. The reflective structure has a characteristic of changing its reflectivity according to a control signal applied thereto.