A planar light source is disclosed. The planar light source includes an upper substrate having a central portion and two edge portions and a lower substrate opposite to the upper substrate, wherein a plurality of luminous cavities are formed by and between the upper substrate and the lower substrate. With respect to cross sectional areas, one portion of the luminous cavities corresponding to the central portion of the upper substrate is smaller than the other portion of the luminous cavities corresponding to one of the edge portions of the upper substrate, and thus the lightness produced in the central portion is higher than that in the edge portion.