An antenna device including a substrate, a ground layer, a first feeding element, a second feeding element, a first control circuit and a second control circuit is provided. The substrate has a top surface and a lower surface. The ground layer disposed on the lower surface includes a first, a second and a third ground portions. The third ground portion is separated from the first and the second ground portions by a first and a second slots, respectively. The first and the second feeding elements include a first and a second conductive feeding lines, respectively. The first and the second conductive feeding lines cross over the first and the second slots and are electrically connected to the first and the second ground portions, respectively. The radiation pattern of the antenna device is variable by selectively operating the first, the second, the third and the fourth control circuits.