An antenna array, comprising: a dielectric substrate, having a first surface and a second surface opposite to each other; a ground metal plane, disposed on the second surface of the dielectric substrate; a first antenna unit, comprising a first metal loop and a first feeding metal element, wherein the first feeding metal element is coupled to a first signal source and is adjacent to the first metal loop; a second antenna unit, comprising a second metal loop and a second feeding metal element, wherein the second feeding metal element is coupled to a second signal source and is adjacent to the second metal loop; a third antenna unit, comprising a third metal loop and a third feeding metal element, wherein the third feeding metal element is coupled to a third signal source and is adjacent to the third metal loop; and a fourth antenna unit, comprising a fourth metal loop and a fourth feeding metal element, wherein the fourth feeding metal element is coupled to a fourth signal source and is adjacent to the fourth metal loop; wherein the first metal loop, the second metal loop, the third metal loop, and the fourth metal loop are disposed on the first surface of the dielectric substrate, wherein the first antenna unit, the second antenna unit, the third antenna unit, and the fourth antenna unit cover a first frequency band and a second frequency band of millimeter-wave operations; wherein the first metal loop has a first hollow portion, the second metal loop has a second hollow portion, the third metal loop has a third hollow portion, the fourth metal loop has a fourth hollow portion, and each of the first hollow portion, the second hollow portion, the third hollow portion, and the fourth hollow portion substantially has a small square shape, and wherein a length of each of the first hollow portion, the second hollow portion, the third hollow portion, and the fourth hollow portion is substantially equal to 0.25 wavelength of the first frequency band. a dielectric substrate, having a first surface and a second surface opposite to each other; a ground metal plane, disposed on the second surface of the dielectric substrate; a first antenna unit, comprising a first metal loop and a first feeding metal element, wherein the first feeding metal element is coupled to a first signal source and is adjacent to the first metal loop; a second antenna unit, comprising a second metal loop and a second feeding metal element, wherein the second feeding metal element is coupled to a second signal source and is adjacent to the second metal loop; a third antenna unit, comprising a third metal loop and a third feeding metal element, wherein the third feeding metal element is coupled to a third signal source and is adjacent to the third metal loop; and a fourth antenna unit, comprising a fourth metal loop and a fourth feeding metal element, wherein the fourth feeding metal element is coupled to a fourth signal source and is adjacent to the fourth metal loop; wherein the first metal loop, the second metal loop, the third metal loop, and the fourth metal loop are disposed on the first surface of the dielectric substrate, wherein the first antenna unit, the second antenna unit, the third antenna unit, and the fourth antenna unit cover a first frequency band and a second frequency band of millimeter-wave operations; wherein the first metal loop has a first hollow portion, the second metal loop has a second hollow portion, the third metal loop has a third hollow portion, the fourth metal loop has a fourth hollow portion, and each of the first hollow portion, the second hollow portion, the third hollow portion, and the fourth hollow portion substantially has a small square shape, and wherein a length of each of the first hollow portion, the second hollow portion, the third hollow portion, and the fourth hollow portion is substantially equal to 0.25 wavelength of the first frequency band.