A substrate is patterned to form trenches and a semiconductor fin between the trenches. Insulators are formed in the trenches and a first dielectric layer is formed to cover the semiconductor fin and the insulators. A dummy gate strip is formed on the first dielectric layer. Spacers are formed on sidewalls of the dummy gate strip. The dummy gate strip and the first dielectric layer underneath are removed until sidewalls of the spacers, a portion of the semiconductor fin and portions of the insulators are exposed. A second dielectric layer is conformally formed to cover the sidewalls of the spacers, the exposed portion of the semiconductor fin and the exposed portions of the insulators, wherein a thickness of the first dielectric layer is smaller than a thickness of the second dielectric layer. A gate is formed on the second dielectric layer and between the spacers.