Methods for fabricating Fin field effect transistors (FinFETs) are disclosed. First and second semiconductor fins and an insulator therebetween are formed. First and second dummy gates and an opening therebetween over the insulator are formed, wherein the first and second dummy gates cross over the first and second semiconductor fins respectively. A first dielectric material with an air gap therein is formed in the opening. A portion of the first dielectric material is removed to expose the air gap, so as to form a first dielectric layer with a slit therein. The first and second dummy gates are removed. A second dielectric layer is formed to fill the slit. First and second gates are formed to cross over portions of the first and second semiconductor fins respectively, wherein the first and second gates are electrically insulated from each other by the first dielectric layer including the second dielectric layer.