Abstract: A planar array antenna structure includes a substrate, an array antenna, and a bottom ground portion. The substrate has a front surface and a rear surface. The array antenna is composed of a plurality of antenna units and disposed on the front surface of the substrate in a symmetrical and polygonal arrangement. A spaced slot is formed between every two antenna units. The bottom ground portion is polygonal and arranged on the rear surface of the substrate. The bottom ground portion has a plurality of included angles thereon, and one notch is formed between two included angles and the notches are correspondingly arranged to the spaced slots. Accordingly, the planar array antenna structure is used to generate high-gain radiation variations, effectively restrain the isolation between the radiators, and significantly increase overall performance of the antenna.