The present disclosure provides a semiconductor device with an air gap between gate-all-around (GAA) transistors and a method for forming the semiconductor device. The semiconductor device includes a first gate stack and a second gate stack disposed over a semiconductor substrate. At least one of the first gate stack and the second gate stack includes a plurality of gate layers, and the first gate stack and the second gate stack have an air gap therebetween. The semiconductor device also includes a first gate structure and a second gate structure disposed over the first gate stack and the second gate stack, respectively, and a first dielectric layer surrounds lower sidewalls of the first gate structure and lower sidewalls of the second gate structure. A width of the first gate structure is greater than a width of the first plug.