A light guide plate (32) for a backlight module includes a light incident surface (321), a bottom surface (323), and a plurality of light guide elements (320) disposed therein. Each light guide element has a light refractive surface (324) opposite to the light incident surface. An angle between the light refractive surface and the bottom surface is an acute angle. A refractive index of each light guide element is different from that of adjacent light guide elements. The reflectivity index of the light guide element and the angle can be selected and arranged so that most if not all the light beams are emitted from the light output surface. Thereby, the light guide plate is highly efficient in the utilization of light beams.