A mobile device includes a ground plane, a grounding branch, a connection element, a first radiation branch, and a second radiation branch. The grounding branch is coupled to the ground plane. A first open slot is formed and substantially surrounded by the grounding branch and the ground plane. The first radiation branch is coupled through the connection element to the grounding branch. A second open slot is formed and is substantially surrounded by the first radiation branch and the grounding branch. The second radiation branch is disposed in the second open slot and is coupled to the grounding branch. A multi-band antenna structure is formed by the grounding branch, the connection element, the first radiation branch, and the second radiation branch.