In a method of manufacturing a semiconductor device, a separation wall made of a dielectric material is formed between two fin structures. A dummy gate structure is formed over the separation wall and the two fin structures. An interlayer dielectric (ILD) layer is formed over the dummy gate structure. An upper portion of the ILD layer is removed, thereby exposing the dummy gate structure. The dummy gate structure is replaced with a metal gate structure. A planarization operation is performed to expose the separation wall, thereby dividing the metal gate structure into a first gate structure and a second gate structure. The first gate structure and the second gate structure are separated by the separation wall.