Semiconductor structures and fabrication methods thereof are provided. An exemplary fabrication method includes providing a base substrate having an N-type logic region including a first and a second N-type threshold voltage region, a P-type logic region including a first and a second P-type threshold voltage region, a pull-up transistor region and an adjacent pull-down transistor region; forming a gate dielectric layer; forming a first work function layer on the gate dielectric layer; removing portions of the first work function layer; forming a second work function layer on remaining first work function layer and exposed portions of the gate dielectric layer; removing a portion of the second work function layer; forming an N-type work function layer on remaining second work function layer and exposed portion of the gate dielectric layer in the second N-type threshold voltage region; and forming a gate electrode layer on the N-type work function layer.