A semiconductor device includes a substrate, a first transistor, a second transistor and a third transistor. The substrate includes a high-voltage (HV) area, a medium-voltage (MV) area, and a low-voltage (LV) area. The first transistor is disposed in the HV area and includes a first gate dielectric layer and a first gate electrode. The second transistor is disposed in the LV area and includes a plurality of fin-shaped structures and a second gate electrode. The third transistor is disposed in the MV area and includes a third gate dielectric layer and a third gate electrode. The topmost surfaces of the first gate electrode, the second gate electrode and the third gate electrode are coplanar with each other.