An Antenna-in-Package (AiP) includes an interface layer having at least an antenna layer and an insulating layer disposed under the antenna layer. The antenna layer includes a first antenna region and a second antenna region spaced apart from the first antenna region. An integrated circuit die is disposed on the interface layer. The integrated circuit die is interposed between the first antenna region and the second antenna region. The first antenna region includes a first antenna element, a second antenna element extending along a first direction, and a feeding network electrically connecting the first antenna element and the second antenna element to the integrated circuit die. The feeding network, the first antenna element, and the second antenna element are coplanar. A plurality of solder balls is disposed on a surface of the interface layer.