A light guide (20) for illuminating keys of a keyboard or keypad of a portable electronic device, includes an array of apertures (22) and a groove (21) at one end of the light guide. The apertures are configured to illuminate the keys via a light source (10). The light source is disposed adjacent to the groove. A peripheral wall around each aperture is configured partially as a reflecting surface, and partially as a transmitting surface. The reflecting angle of the reflecting surface is larger than a critical total internal reflection angle so as to transfer the light farther. The light guide has a desired uniform light by means of a light source. This provides the portable electronic device consuming little power.