An illumination system includes at least an LED module, and at least an illuminated area. The LED module includes an LED, and a lens mounted in light path of the LED. The lens includes a light source recess, a first light emitting surface, a critical reflecting surface, and a second light emitting surface. The first light emitting surface can receive more light quantity than the second light emitting surface. In result, although the light emitted from the first light emitting surface may have greater attenuation than the light emitted from the second light emitting surface, light emitted from the first light emitting surface, which is father to the illuminated are, can make up the intensity losses of attenuation as the first light emitting surface receives more light quantity than the second light emitting surface. As a result, the illumination system has a uniform illumination pattern.