A semiconductor device includes a substrate, a first semiconductor fin, a second semiconductor fin, a gate structure, a plurality of source/drain structures, a shallow trench isolation (STI) oxide, and a dielectric layer. The first semiconductor fin extends upwardly from the substrate. The second semiconductor fin extends upwardly from the substrate. The gate structure extends across the first and second semiconductor fins. The source/drain structures are on the first and second semiconductor fins. The STI oxide extends continuously between the first and second semiconductor fins and has a U-shaped profile when viewed in a cross section taken along a lengthwise direction of the gate structure. The dielectric layer is partially embedded in the STI oxide and has a U-shaped profile when viewed in the cross section taken along the lengthwise direction of the gate structure.