An exemplary backlight module includes a frame (16) and a light guide plate (14) positioned in the frame. The frame defines a notch (164), which notch is bounded by a wall of the frame. The light guide plate includes a side surface (141), and an ear (144) extending from the side surface. The ear is received in the notch. A first protrusion (165) protrudes from either the wall of the notch or the ear and contacts the corresponding ear or wall of the notch.