This present invention discloses a backlight module including a frame and a light guide plate. The frame includes a base plane and a side wall, wherein the side wall is disposed at the edge of the base plane and encloses a disposition space. The side wall includes a first inner surface and an outer surface, wherein a distance between the first inner surface and the outer surface is decreased as the first inner surface comes closer to a bottom of the side wall. The light guide plate is disposed on the base plane and within the disposition space, wherein the first inner surface sinks toward the outer surface and a recessed space is formed between the first inner surface and a lateral side of the light guide plate.