A mobile device includes a ground element, a radiation element, a first short-circuited element, a second short-circuited element, and a switch element. The radiation element has a feeding point, a fixed grounding point, and a switchable grounding point. The fixed grounding point is coupled through the first short-circuited element to the ground element. The switchable grounding point is coupled through the second short-circuited element and the switch element to the ground element. An antenna structure is formed by the radiation element, the first short-circuited element, the second short-circuited element, and the switch element.