A planar light source including an LGP and first and second light sources are provided. The LGP has a first surface and a second surface opposite to each other and a first groove and a second groove. The first groove and the second groove are respectively located on the first surface and the second surface, and are respectively located at two sides of a center line of the LGP, where the center line equally divides the first surface and the second surface. The first groove has a first side surface and a first light incident surface, and the second groove has a second side surface and a second light incident surface, where the light sources are disposed in the grooves. The light sources are suitable for respectively providing a light beam entering the LGP through the corresponding light incident surface.