A backlight module (20) includes a light source (21), a prism sheet (23) and a light guide plate (22). The light guide plate includes an incident surface (221), an emitting surface (223), a bottom surface (222) and a plurality of depressions (224). The emitting surface is located adjoining the incident surface. The depressions are formed on the emitting surface. Each depression has a refractive surface (225) and a reflective surface (226) adjoining the refractive surface. The reflective surface faces the incident surface, and the refractive surface faces away from the incident surface. The light source is arranged besides the light guide plate and facing the incident surface. The prism sheet has a prism surface facing the emitting surface of the light guide plate and forms a plurality of prisms (241) on the prism surface.