A flat panel antenna includes a horizontal ground plan, a vertical ground plane, a radiation metal member disposed over and in parallel with the horizontal ground plane; the vertical ground plane being disposed at a level higher than that of the radiation metal member, an extension member extending from the vertical ground plane in the direction of and in parallel with the radiation metal member to penetrate into the radiation metal member for a proper length for precise control impedance of the antenna for the normalized impedance to approach 1, i.e. 50-ohm of the system impedance of the antenna to upgrade radiation efficiency of the flat panel antenna.