An antenna structure includes a main radiation element, a first feeding element, a first additional radiation element, a dielectric substrate, and a ground plane. A first signal source is coupled through the first feeding element to a first side of the main radiation element. The first additional radiation element is coupled to a second side of the main radiation element. A first slot is formed between the first additional radiation element and the main radiation element. The second side is different from the first side. The dielectric substrate has a first surface and a second surface which are opposite to each other. The main radiation element, the first feeding element, and the first additional radiation element are disposed on the first surface of the dielectric substrate. The ground plane is adjacent to the second surface of the dielectric substrate.