A planar directional antenna including a substrate, a metal layer, a master antenna, and an auxiliary antenna is provided. The substrate has a first surface and a second surface. The metal layer is disposed on the second surface of the substrate, and an upper edge of the metal layer forms a concave parabolic curve. The master antenna is disposed on the substrate and located within a predetermined range of the focus of the concave parabolic curve. The auxiliary antenna is disposed on the substrate and opposite to the master antenna so that the planar directional antenna generates a beam toward a radiation direction.