A surface light source (4) includes a light guide plate (2) and a light source (5). The light guide plate has an incident surface (21) and an emitting surface (22). The light source is adjacent to the incident surface of the light guide plate for radiating light beams into the light guide plate through the incident surface. The emitting surface defines a large number of prisms (221), in which the distance (L) separating each two adjacent prisms changes according to an intensity of the light beams received from the light source. In particular, the pitch between two prisms decreases with increasing distance away from the light source. By the cooperation of the prisms and the light source, the surface light source has uniform illumination over the whole emitting surface.