A semiconductor structure includes a substrate, at least one active semiconductor fin, at least one insulating structure, a gate electrode, and a gate dielectric. The active semiconductor fin is disposed on the substrate. The insulating structure is disposed on the substrate and adjacent to the active semiconductor fin. A top surface of the insulating structure is non-concave and is lower than a top surface of the active semiconductor fin. The gate electrode is disposed over the active semiconductor fin. The gate dielectric is disposed between the gate electrode and the active semiconductor fin.