A back light system (2) includes a light guide plate (20) and light sources (21). The light guide plate includes an optical output surface (203), a bottom surface (202) opposite to the output surface, and optical input surfaces (201) between the output and bottom surfaces. The light sources are positioned adjacent the input surfaces respectively. A thickness of a middle of the light guide plate is less than a thickness of a periphery of the light guide plate. In operation, light beams from two opposite of the light sources provide ample optical energy to a middle area of the output surface. In addition, a total optical energy at peripheral areas of the output surface is the same as or similar to a total optical energy at the middle area of the output surface. Thus, the back light system provides very high and highly uniform brightness for an associated LCD.