An antenna structure includes a ground plane, a first radiation element, a second radiation element, a third radiation element, and a fourth radiation element. A closed slot is formed in the ground plane. The first radiation element has a feeding point. The first radiation element is coupled to a first shorting point on the ground plane. The second radiation element is coupled to a second shorting point on the ground plane. The second radiation element is adjacent to the first radiation element. The third radiation element is coupled to the feeding point. The fourth radiation element is coupled to a third shorting point on the ground plane. The fourth radiation element is adjacent to the third radiation element. The first radiation element, the second radiation element, the third radiation element, and the fourth radiation element are all disposed inside the closed slot.