A planar array antenna structure comprising: a substrate having a front surface and a rear surface; and an array antenna composed of a plurality of antenna units and disposed on the front surface of the substrate; wherein the antenna units are disposed on the front surface of the substrate in a symmetrical and polygonal arrangement, and a spaced slot is formed between every two antenna units, wherein the polygonal arrangement is a triangular arrangement, a quadrilateral arrangement, a pentagonal arrangement, a hexagonal arrangement, a heptagonal arrangement, or an octagonal arrangement, wherein each antenna unit has a top ground portion, a main radiator, and an auxiliary radiator, wherein the top ground portion is trapezoidal and has an upper edge and a lower edge, wherein the main radiator is rectangular or square, and the main radiator having a signal feed point is arranged at left of the lower edge of the top ground portion; the signal feed point is electrically connected to the lower edge of the top ground portion, wherein the auxiliary radiator is L-shaped, and arranged at right of the lower edge of the top ground portion and electrically connected to the lower edge, a bottom ground portion disposed on the rear surface of the substrate and corresponding to the antenna units disposed on the front surface of the substrate; the bottom ground portion is polygonal, such as triangular, quadrilateral, pentagonal, hexagonal, heptagonal, or octagonal; the bottom ground portion has a plurality of included angles thereon, and a notch is formed between two included angles and the notches are correspondingly arranged to the spaced slots formed on the front surfaces of the substrate, wherein the spaced slots and the notches are configured to generate an optimum resonance current to restrain the current generated from adjacent main radiators and auxiliary radiators to achieve the best isolation. a substrate having a front surface and a rear surface; and an array antenna composed of a plurality of antenna units and disposed on the front surface of the substrate; wherein the antenna units are disposed on the front surface of the substrate in a symmetrical and polygonal arrangement, and a spaced slot is formed between every two antenna units, wherein the polygonal arrangement is a triangular arrangement, a quadrilateral arrangement, a pentagonal arrangement, a hexagonal arrangement, a heptagonal arrangement, or an octagonal arrangement, wherein each antenna unit has a top ground portion, a main radiator, and an auxiliary radiator, wherein the top ground portion is trapezoidal and has an upper edge and a lower edge, wherein the main radiator is rectangular or square, and the main radiator having a signal feed point is arranged at left of the lower edge of the top ground portion; the signal feed point is electrically connected to the lower edge of the top ground portion, wherein the auxiliary radiator is L-shaped, and arranged at right of the lower edge of the top ground portion and electrically connected to the lower edge, a bottom ground portion disposed on the rear surface of the substrate and corresponding to the antenna units disposed on the front surface of the substrate; the bottom ground portion is polygonal, such as triangular, quadrilateral, pentagonal, hexagonal, heptagonal, or octagonal; the bottom ground portion has a plurality of included angles thereon, and a notch is formed between two included angles and the notches are correspondingly arranged to the spaced slots formed on the front surfaces of the substrate, wherein the spaced slots and the notches are configured to generate an optimum resonance current to restrain the current generated from adjacent main radiators and auxiliary radiators to achieve the best isolation.