A light guide plate (30) includes an emitting surface (32) having a plurality of V-shaped prisms (33) thereat, a bottom surface (34) opposite to the emitting surface, the bottom surface having a plurality of arc-shaped grooves (35) thereon, and a cutout portion at a corner between two adjacent sides of the light guide plate, the cutout portion defining a light incident surface (31). When light beams enter the light guide plate and are reflected by the arc-shaped grooves, the parallel V-shaped prisms significantly reduce or even eliminate further reflection of the reflected light back into the light guide plate. Most or all reflected light beams reaching the emitting surface transmits out from the light guide plate and are utilized. Therefore, the light guide plate and the backlight module can provide highly efficient utilization of light beams.