A light guide plate (12) has a light incidence surface (121) for receiving light, a light emitting surface (123) for emitting light, and a bottom surface (122). The bottom surface has a plurality of diffusion elements (124) arranged thereat. Each diffusion element defines a diffraction grating unit (125) therein. A grating direction of each diffraction grating unit is substantially perpendicular to a main direction of light beams received by the diffraction grating unit. Areas of the diffraction grating units progressively increase with increasing distance away from the light incidence surface. Diffractive capabilities of the diffraction grating units progressively increase with increasing distance away from the light incidence surface. These features improve the overall efficiency of utilization of light, and enable the light emitting surface to output highly uniform light.