A plane light source has a light source (50), a reflection cover (51) and a light guide plate (10). The light guide plate has a wedge-shaped base plate (20), an array of reflection lumps (30), and an array of round protrusions (40). The base plate has a light incidence surface (21), a light emitting surface (23), and a bottom surface (25) opposite to the light emitting surface. The reflection lumps are disposed on the bottom surface, and penetrate into the base plate. The round protrusions are disposed on the light emitting surface for diffusing light. Each reflection lump corresponds to a respective one of the round protrusions. Each reflection lump has a reflecting surface (301) for reflecting incident light to the corresponding round protrusion of the light emitting surface.