A backlight module includes a light guide plate and at least one light source. The light guide plate has a first light-emitting surface, a second light-emitting surface opposite the first light-emitting surface, and at least one side surface connected between the first light-emitting surface and the second light-emitting surface. The light guide plate has a first substance and a second substance surrounding the first substance, and the second substance is different to the first substance to form at least a first light reflecting/diffusing interface and a second light reflecting/diffusing interface. A light beam emitted by the light source is deflected by the first light reflecting/diffusing interface and output via the first light-emitting surface, and a light beam emitted by the light source is deflected by the second light reflecting/diffusing interface and output via the second light-emitting surface.