A light guide plate (20) in accordance with the present invention includes a transparent plate (200) and a light transmittance enhancement layer (25). The transparent plate includes a light emitting surface (22), and a light incidence surface (21) opposite to the light emitting surface. The light transmittance enhancement layer is provided on the light emitting surface. The light transmittance enhancement layer is made of silicon dioxide or magnesium fluoride. A thickness of the light transmittance enhancement layer is in the range from 58˜69 nanometers. Because of the light transmittance enhancement layer, the light guide plate has enhanced efficiency of utilization of visible light. A backlight system (2) using the light guide plate is also provided.